Properly typecheck types.http (#14988)

* Tweak http types in Synapse

AFACIS these are correct, and they make mypy happier on tests.http.

* Type hints for test_proxyagent

* type hints for test_srv_resolver

* test_matrix_federation_agent

* tests.http.server._base

* tests.http.__init__

* tests.http.test_additional_resource

* tests.http.test_client

* tests.http.test_endpoint

* tests.http.test_matrixfederationclient

* tests.http.test_servlet

* tests.http.test_simple_client

* tests.http.test_site

* One fixup in tests.server

* Untyped defs

* Changelog

* Fixup syntax for Python 3.7

* Fix olddeps syntax

* Use a twisted IPv4 addr for dummy_address

* Fix typo, thanks Sean

Co-authored-by: Sean Quah <8349537+squahtx@users.noreply.github.com>

* Remove redundant `Optional`

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Co-authored-by: Sean Quah <8349537+squahtx@users.noreply.github.com>