pycompat: deprecate using bytes
Python2 has been dropped for a while, so lets comply to the signature of the
global function.
This open the way to drop the use of `pycompat.getattr` and company, and,
especially, the associated `util.safehasattr`.
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https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
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273ce12ad8f1` will report some style violations on a very old commit).