Add type hints for checking by mypy

[H-Shay]

Here is the first round of adding support for mypy. A few things:

• This PR resulted from my adding all the type annotations in the comments to various function and getting mypy to recognize them
• I started trying to fix the errors that adding types produced but at some point realized it was too many changes at once/some of the changes were structural in ways I didn't feel confident about changing/etc. So I figured lets just get these baseline changes approved and merged and then go from there-i am sure there will be revisions
• a lot of these changes were re-written as I learned more (using optional[whatever] instead of union[none, whatever], etc) so the final changes are in last committed version of each file
• there are some files that already had annotations that I did not add, i did not change those
• there is a directory http/servlets that I did not do, and i did not do sydent.py because it seemed like this PR was getting huge so i tried to stop somewhere that i could be clear on where to start again

signed off by H-Shay: shaysquared@gmail.com

[callahad]

This is great! I have a few questions / suggested tweaks, and it looks like the black lint isn't passing (mainly Black wanting double quotes in places).

The unpaddedbase64 requirement in setup.py can be updated to >=1.1.0 to pull in a version which supplies type hints, avoiding the need for some type: ignore lines (best done in a followup pull request to ease review)

Generally, type: ignore lines are probably better replaced with a mypy.ini config file and set like:

[mypy-signedjson]
ignore_missing_imports = True

[mypy-unpaddedbase64]
ignore_missing_imports = True

Which has the same effect, but without all the individual annotations

[H-Shay]

This last commit should take care of the requested changes on this branch that aren't going to be moved to a separate PR. Thanks again for reviewing!

[callahad]

Looks like flake8 in CI isn't totally happy yet, otherwise this is good to go once these are fixed:

sydent/http/httpsclient.py:21:1: F401 'typing.Generator' imported but unused
sydent/http/httpsclient.py:27:1: F401 'twisted.web.iweb.IResponse' imported but unused
sydent/http/srvresolver.py:24:1: F401 'twisted.internet.defer.Deferred' imported but unused
sydent/db/threepid_associations.py:19:1: F401 'typing.Sequence' imported but unused
sydent/db/hashing_metadata.py:20:1: F401 'typing.Union' imported but unused
sydent/util/ttlcache.py:18:1: F401 'typing.Dict' imported but unused
sydent/sms/openmarket.py:20:1: F401 'typing.Any' imported but unused
sydent/threepid/bind.py:22:1: F401 'typing.Generator' imported but unused
sydent/threepid/bind.py:22:1: F401 'typing.Optional' imported but unused
sydent/replication/peer.py:22:1: F401 'typing.Generator' imported but unused

[callahad]

Woohoo! This is an awesome step toward more confidence in refactoring Sydent. Thank you!