typing: add type hints to `mercurial.policy`...
typing: add type hints to `mercurial.policy`
Mostly trivial, but this seems like the logical module to use to inject the
hints from `cext`, `pure`, etc, given that this file has the fallback policy.
This is a first step.
There doesn't appear to be a predefined type for a module in py3.7, so those are
omitted for now.
