typing: add trivial type hints to the convert extension's common modules
This started as ensuring that the `encoding` and `orig_encoding` attributes has
a type other than `Any`, so pytype can catch problems where it needs to be str
for stdlib encoding and decoding. It turns out that adding the hint in
`mercurial.encoding` is what was needed, but I picked a bunch of low hanging
fruit while here. There's definitely more to do, and I see a problem where
`shlex.shlex` is being fed bytes instead of str, but there are not enough type
hints yet to make pytype notice.
(binary:application/octet-stream)