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.

this is part of _retry_on_integrity_error, so should only be on _persist_events_and_state_updates

Move persist_events out from main data store.

Make ObservableDeferred.observe() always return deferred.