typing: add a type hint to `mercurial/hg.py` Somewhere between hg 3dbc7b1ecaba and hg 8e3f6b5bf720, the first value of the tuple changed from bytes to str. Let's lock this in, so that pytype flags it if someone mistakenly adds a tuple with bytes somewhere.

typing: restore `encoding.encoding` and `encoding.encodingmode` to bytes Somewhere between hg 3dbc7b1ecaba and hg 8e3f6b5bf720, pytype determined the signature of these fields changed from `bytes` to `Any`. Not sure why- the type of `environ` then and now is: `Union[WindowsEnviron, Dict[bytes, bytes], os._Environ[bytes]]` That said, PyCharm wasn't able to figure out the type of `environ`, and the `WindowsEnviron` class extends `MutableMapping` without specifying bytes for the key and value types in py3.9. But that's not changed in my setup, so I can't explain it.

typing: add some trivial type hints to `mercurial/bundlecaches.py` The function is meant for extensions, but it wasn't obvious what was expected without looking through the code. Also, pytype couldn't figure it out either.