squash! Add type inference for dict.keys membership

Move tests under `test-data/unit/pythoneval.test` this is to avoid me
having to add test stubs handling `dict.keys` which was rather difficult
because:

* The implementation requires me importing `dict_keys` from
  `_collections_abc` in `test-data/unit/fixtures/dict.pyi`
* Since there aren't any stubs for this, a large number of tests would
  start failing, e.g.

    $ pytest -q mypy/test/testcheck.py::TypeCheckSuite::check-incremental.test::testIncrementalWithDifferentKindsOfNestedTypesWithinMethod
    F                                                                                                                                                       [100%]
    ========================================================================== FAILURES ===========================================================================
    _________________________________________________ testIncrementalWithDifferentKindsOfNestedTypesWithinMethod __________________________________________________
    data: /home/mjh/src/mypy/test-data/unit/check-incremental.test:5662:
    /home/mjh/src/mypy/mypy/test/testcheck.py:80: in run_case
        self.run_case_once(testcase, ops, step)
    /home/mjh/src/mypy/mypy/test/testcheck.py:174: in run_case_once
        assert_string_arrays_equal(output, a, msg.format(testcase.file, testcase.line))
    E   AssertionError: Unexpected type checker output in incremental, run 2 (/home/mjh/src/mypy/test-data/unit/check-incremental.test, line 5662)
    -------------------------------------------------------------------- Captured stderr call ---------------------------------------------------------------------
    Expected:
      tmp/a.py:2: error: "object" has no attribute "xyz" (diff)
    Actual:
      tmp/a.py:2: error: Module has no attribute "xyz" (diff)

    Alignment of first line difference:
      E: tmp/a.py:2: error: "object" has no attribute "xyz"
      A: tmp/a.py:2: error: Module has no attribute "xyz"
                            ^
    =================================================================== short test summary info ===================================================================
    FAILED mypy/test/testcheck.py::TypeCheckSuite::check-incremental.test::testIncrementalWithDifferentKindsOfNestedTypesWithinMethod
    1 failed in 0.33s

* If I _were_ to add `test-data/unit/lib-stub/_collections_abc.pyi` then
  tests importing from `collections.abc` start failing, e.g.

    ____________________________________________________________ testGenericAliasCollectionsABCReveal _____________________________________________________________
    data: /home/mjh/src/mypy/test-data/unit/check-generic-alias.test:147:
    /home/mjh/src/mypy/mypy/test/testcheck.py:82: in run_case
        self.run_case_once(testcase)
    /home/mjh/src/mypy/mypy/test/testcheck.py:174: in run_case_once
        assert_string_arrays_equal(output, a, msg.format(testcase.file, testcase.line))
    E   AssertionError: Unexpected type checker output (/home/mjh/src/mypy/test-data/unit/check-generic-alias.test, line 147)
    -------------------------------------------------------------------- Captured stderr call ---------------------------------------------------------------------
    Expected:
    Actual:
      main:6: error: Name "collections.abc.AsyncIterable" is not defined (diff)
      main:7: error: Name "collections.abc.AsyncIterator" is not defined (diff)
      main:8: error: Name "collections.abc.AsyncGenerator" is not defined (diff)
      main:12: error: Name "collections.abc.Reversible" is not defined (diff)
      main:13: error: Name "collections.abc.Container" is not defined (diff)
      main:14: error: Name "collections.abc.Collection" is not defined (diff)
      main:16: error: Name "collections.abc.Set" is not defined (diff)
      main:17: error: Name "collections.abc.MutableSet" is not defined (diff)
      main:19: error: Name "collections.abc.MutableMapping" is not defined (diff)
      main:21: error: Name "collections.abc.MutableSequence" is not defined (diff)
      main:22: error: Name "collections.abc.ByteString" is not defined (diff)
      main:23: error: Name "collections.abc.MappingView" is not defined (diff)
      main:24: error: Name "collections.abc.KeysView" is not defined (diff)
      main:25: error: Name "collections.abc.ItemsView" is not defined (diff)
      main:26: error: Name "collections.abc.ValuesView" is not defined (diff)

Author: matthewhughes934

test-data/unit/check-isinstance.test

@@ -2065,19 +2065,6 @@ def f() -> None:
 [builtins fixtures/dict.pyi]
 [out]
 
-[case testNarrowTypeAfterInDictKeys]
-# flags: --strict-optional
-from typing import Dict, Optional
-
-def f() -> None:
-   d: Dict[str, str] = {}
-   key: Optional[str]
-   if key in d.keys():
-       reveal_type(key) # N: Revealed type is "builtins.str"
-   else:
-       reveal_type(key) # N: Revealed type is "Union[builtins.str, None]"
-[builtins fixtures/dict.pyi]
-
 [case testIsinstanceWidensWithAnyArg]
 from typing import Any
 class A: ...

test-data/unit/fixtures/dict.pyi

@@ -1,6 +1,5 @@
 # Builtins stub used in dictionary-related test cases.
 
-from _collections_abc import dict_keys
 from typing import (
     TypeVar, Generic, Iterable, Iterator, Mapping, Tuple, overload, Optional, Union, Sequence
 )
@@ -31,7 +30,6 @@ class dict(Mapping[KT, VT]):
     @overload
     def get(self, k: KT, default: Union[KT, T]) -> Union[VT, T]: pass
     def __len__(self) -> int: ...
-    def keys(self) -> dict_keys[KT, VT]: ...
 
 class int: # for convenience
     def __add__(self, x: Union[int, complex]) -> int: pass

test-data/unit/pythoneval.test

@@ -1612,3 +1612,15 @@ class Foo(Enum):
 _testEnumValueWithPlaceholderNodeType.py:5: error: Incompatible types in assignment (expression has type "object", variable has type "Foo")
 _testEnumValueWithPlaceholderNodeType.py:6: error: Incompatible types in assignment (expression has type "object", variable has type "Foo")
 _testEnumValueWithPlaceholderNodeType.py:6: error: Name "Missing" is not defined
+
+[case testNarrowTypeAfterInDictKeys]
+# flags: --strict-optional
+from typing import Dict, Optional
+
+def f() -> None:
+   d: Dict[str, str] = {}
+   key: Optional[str]
+   if key in d.keys():
+       reveal_type(key) # N: Revealed type is "builtins.str"
+   else:
+       reveal_type(key) # N: Revealed type is "Union[builtins.str, None]"