Fix annotations of CallList

[ziima]

With 0.22.0, mypy started to complain about constructions rsps.calls[0].request with an error

Item "List[Call]" of "Union[Call, List[Call]]" has no attribute "request"  [union-attr]

because it wasn't able to differentiate between the return types. I added overload annotation to handle that.

[beliaev-maksim]

hi, thanks

please add new version in changelog and add your item there

[ziima]

Done.

[markstory]

Thank you 🎉

[andersk]

When you get a chance, it’d be great to get a new PyPI release, since none of the existing releases work with mypy (#552 for ≤ 0.20.0, #556 for 0.21.0, this for 0.22.0) and current Git works.

[beliaev-maksim]

closes #610