fixup! Add type inference for dict.keys membership

Author: matthewhughes934

test-data/unit/check-isinstance.test

@@ -2072,9 +2072,10 @@ from typing import Dict, Optional
 def f() -> None:
    d: Dict[str, str] = {}
    key: Optional[str]
-   if key not in d.keys():
-        return
-   reveal_type(key) # N: Revealed type is "builtins.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]