Run self-check on Windows as part of CI (#11909)

As discussed in #11895, mypy's test suite currently does not pass a mypy self-check when run on Windows. This should hopefully be fixed by python/typeshed#6812, but running a self-check on Windows as part of the CI tests should help avoid this issue in the future.

Author: AlexWaygood

.github/workflows/test.yml

@@ -75,11 +75,16 @@ jobs:
           toxenv: py36
           tox_extra_args: "-n 2 mypyc/test/test_run.py mypyc/test/test_external.py"
           debug_build: true
-        - name: Type check our own code
+        - name: Type check our own code (py37-ubuntu)
           python: '3.7'
           arch: x64
           os: ubuntu-latest
           toxenv: type
+        - name: Type check our own code (py37-windows-64)
+          python: '3.7'
+          arch: x64
+          os: windows-latest
+          toxenv: type
         - name: Code style with flake8
           python: '3.7'
           arch: x64

mypy/util.py

@@ -517,6 +517,8 @@ def hash_digest(data: bytes) -> str:
 
 def parse_gray_color(cup: bytes) -> str:
     """Reproduce a gray color in ANSI escape sequence"""
+    if sys.platform == "win32":
+        assert False, "curses is not available on Windows"
     set_color = ''.join([cup[:-1].decode(), 'm'])
     gray = curses.tparm(set_color.encode('utf-8'), 1, 89).decode()
     return gray
@@ -580,7 +582,7 @@ def initialize_win_colors(self) -> bool:
 
     def initialize_unix_colors(self) -> bool:
         """Return True if initialization was successful and we can use colors, False otherwise"""
-        if not CURSES_ENABLED:
+        if sys.platform == "win32" or not CURSES_ENABLED:
             return False
         try:
             # setupterm wants a fd to potentially write an "initialization sequence".