When the `/keys/query` API is hit on client_reader worker Synapse may
decide that it needs to resync some remote deivces. Usually this happens
on master, and then gets cached. However, that fails on workers and so
it falls back to fetching devices from remotes directly, which may in
turn fail if the remote is down.

* remove psutil and replace with resource

since I found myself wonder how it works

Implement MSC2326 (label based filtering)

The intention here is to make it clearer which fields we can expect to be
populated when: notably, that the _event_type etc aren't used for the
synchronous impl of EventContext.

Co-Authored-By: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

fix hidden field in devices table for older sqlite

* Add lint dependencies black, flake8 and isort

These are required when running the `lint.sh` dev scripts.

Signed-off-by: Jason Robinson <jasonr@matrix.org>

* Add contributer docs for using the providers linters script

Add also to the pull request template to avoid build failures due
to people not knowing that linters need running.

Signed-off-by: Jason Robinson <jasonr@matrix.org>

* Fix mention of linter errors correction

Co-Authored-By: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

* Add mention for installing linter dependencies

Co-Authored-By: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

* Remove linters from python dependencies as per PR review

Signed-off-by: Jason Robinson <jasonr@matrix.org>

The `http_proxy` and `HTTPS_PROXY` env vars can be set to a `host[:port]` value which should point to a proxy.

The address of the proxy should be excluded from IP blacklists such as the `url_preview_ip_range_blacklist`.

The proxy will then be used for
 * push
 * url previews
 * phone-home stats
 * recaptcha validation
 * CAS auth validation

It will *not* be used for:
 * Application Services
 * Identity servers
 * Outbound federation
 * In worker configurations, connections from workers to masters

Fixes #4198.

Cross-signing [4/4] -- federation edition

Co-Authored-By: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

make notification of signatures work with workers

* update version of black and also fix the mypy config being overridden

Add StateGroupStorage interface

* Offer the homeserver instance to the spam checker

* Newsfile

* Linting

* Expose a Spam Checker API instead of passing the homeserver object

* Alter changelog

* s/hs/api

Fix /purge_room admin API

Fix docstring

Co-Authored-By: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

It fails trying to clean the `topic` table which was recently removed.

Replace every instance of `logger.warn` with `logger.warning` as the former is deprecated.