Exclude Y037 in .flake8 config, for now (#7600)

Unblocks PyCQA/flake8-pyi#202

.flake8

@@ -25,16 +25,19 @@
 #       Y034 Detect common errors where certain methods are annotated as having a fixed
 #            return type, despite returning self at runtime (temporarily disabled for
 #            typing.pyi, enabled elsewhere).
+#       Y037 Use PEP 604 syntax instead of `typing.Union` and `typing.Optional`.
+#            Currently can't be enabled due to a few lingering bugs in mypy regarding
+#            PEP 604 type aliases (see #4819).
 
 [flake8]
 per-file-ignores =
   *.py: E203, E501
-  *.pyi: E301, E302, E305, E501, E701, E741, F401, F403, F405, F822, Y026
+  *.pyi: E301, E302, E305, E501, E701, E741, F401, F403, F405, F822, Y026, Y037
   # Since typing.pyi defines "overload" this is not recognized by flake8 as typing.overload.
   # Unfortunately, flake8 does not allow to "noqa" just a specific error inside the file itself.
   # https://github.com/PyCQA/flake8/issues/1079
   #     F811 redefinition of unused '...'
-  stubs/*.pyi: E301, E302, E305, E501, E701, E741, F401, F403, F405, F822, Y026, Y027
-  stdlib/typing.pyi: E301, E302, E305, E501, E701, E741, F401, F403, F405, F811, F822, Y026, Y034
+  stubs/*.pyi: E301, E302, E305, E501, E701, E741, F401, F403, F405, F822, Y026, Y027, Y037
+  stdlib/typing.pyi: E301, E302, E305, E501, E701, E741, F401, F403, F405, F811, F822, Y026, Y034, Y037
 
 exclude = .venv*,.git,*_pb2.pyi,stdlib/@python2/*