Instead of parsing the full response to `/send_join` into Python objects (which can be huge for large rooms) and *then* parsing that into events, we instead use ijson to stream parse the response directly into `EventBase` objects.

This will double count slightly in the presence of interned strings. It's off by default as it can consume a lot of resources.

I went through and removed a bunch of cruft that was lying around for compatibility with old Python versions. This PR also will now prevent Synapse from starting unless you're running Python 3.6+.

Running `dmypy run` will do a `mypy` check while spinning up a daemon
that makes rerunning `dmypy run` a lot faster.

`dmypy` doesn't support `follow_imports = silent` and has
`local_partial_types` enabled, so this PR enables those options and
fixes the issues that were newly raised. Note that `local_partial_types`
will be enabled by default in upcoming mypy releases.

This great big stack of commits is a a whole load of hoop-jumping to make it easier to store additional values in login tokens, and then to actually store the SSO Identity Provider in the login token. (Making use of that data will follow in a subsequent PR.)

This finishes adding type hints to the `synapse.handlers` module.

With this change all handlers except the e2e_* ones have
type hints enabled.

... and disable coverage tracking for mypy and friends.

This improves type hinting and should use less memory.

* SsoHandler: remove inheritance from BaseHandler

* Simplify the flow for SSO UIA

We don't need to do all the magic for mapping users when we are doing UIA, so
let's factor that out.

These things seemed to be completely untested, so I added a load of tests for
them.

This also removes some duplicated code between the simple
HTTP client and matrix federation client.

This also expands type-hints to the SSO and registration code.

Refactors the CAS code to more closely match OIDC/SAML.

Fixes #4042

This also fixes a bug by fixing handling of an account which doesn't expire.

Optionally sends typing, presence, and read receipt information to appservices.

This seemed to entail dragging in a type stub for SortedList.