This avoids these false warnings:
File "/mnt/c/Users/Matt/hg/mercurial/utils/procutil.py", line 78, in <module>: No attribute 'register' on Type[io.BufferedIOBase] [attribute-error] File "/mnt/c/Users/Matt/hg/mercurial/utils/procutil.py", line 117, in <module>: No attribute 'register' on Type[io.IOBase] [attribute-error] File "/mnt/c/Users/Matt/hg/mercurial/utils/procutil.py", line 770, in runbgcommandpy3: No attribute 'close' on int [attribute-error] In Union[IO[Union[bytes, str]], int]