Pin flake8 & pycodestyle; fix style issues

Author: 1st1

.flake8

@@ -1,3 +1,3 @@
 [flake8]
-ignore = E402,E731
+ignore = E402,E731,W504,E252
 exclude = .git,__pycache__,build,dist,.eggs,.github,.local

asyncpg/pool.py

@@ -304,13 +304,14 @@ class Pool:
     Pools are created by calling :func:`~asyncpg.pool.create_pool`.
     """
 
-    __slots__ = ('_queue', '_loop', '_minsize', '_maxsize',
-                 '_init', '_connect_args', '_connect_kwargs',
-                 '_working_addr', '_working_config', '_working_params',
-                 '_holders', '_initialized', '_initializing', '_closing',
-                 '_closed', '_connection_class', '_generation',
-                 '_setup', '_max_queries', '_max_inactive_connection_lifetime'
-                )
+    __slots__ = (
+        '_queue', '_loop', '_minsize', '_maxsize',
+        '_init', '_connect_args', '_connect_kwargs',
+        '_working_addr', '_working_config', '_working_params',
+        '_holders', '_initialized', '_initializing', '_closing',
+        '_closed', '_connection_class', '_generation',
+        '_setup', '_max_queries', '_max_inactive_connection_lifetime'
+    )
 
     def __init__(self, *connect_args,
                  min_size,

setup.py

@@ -31,7 +31,11 @@
 
 # Minimal dependencies required to test asyncpg.
 TEST_DEPENDENCIES = [
-    'flake8~=3.5.0',
+    # pycodestyle is a dependency of flake8, but it must be frozen because
+    # their combination breaks too often
+    # (example breakage: https://gitlab.com/pycqa/flake8/issues/427)
+    'pycodestyle~=2.5.0',
+    'flake8~=3.7.9',
     'uvloop~=0.14.0;platform_system!="Windows"',
 ]