typing: fix the hint for the `mode` argument of `platform.copymode()`
The posix module is doing a bitwise AND with this and an integer, so it can't be
bytes. The only caller that provides the argument is `util.mktempcopy()`, and
pytype infers the type as Any, which explains why this wasn't caught.
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).