typing: suppress a warning that mercurial.windows.checkosfilename is missing

This function is used to check filename portability everwhere, so it isn't
tucked into the windows.py module.  I supposed the alternative is to move it and
then alias it in `util`.  I'm guessing it was done like this to save an import.

Differential Revision: https://phab.mercurial-scm.org/D7677

mercurial/util.py

@@ -2050,7 +2050,8 b' if pycompat.iswindows:'
     checkosfilename = checkwinfilename
     timer = time.clock
 else:
-    checkosfilename = platform.checkosfilename
+    # mercurial.windows doesn't have platform.checkosfilename
+    checkosfilename = platform.checkosfilename  # pytype: disable=module-attr
     timer = time.time
 
 if safehasattr(time, "perf_counter"):