Pyramid-heroku

-----

* Handle IPv6 in client_addr tween. ClientAddr tween was crashing when getting
IPv6 addresses. Use IP version agnostic parsing. Fetch the list of
CloudFlare's IPv6 IPs.
[am-on]

-----

* Don't use Cloudflare IPs for client_addr. ClientAddr tween now sets a
correct IP when using Cloudflare's reverse proxy. The tween fetches the list
of Cloudflare's IPs on pyramid app start and ignores Cloudflare's IPs in
`X-Forwarded-For` header.
[am-on]

* Bump year to 2022
[am-on]

* Rename niteoweb -> teamniteo
[zupo]

-----

* Do not touch Heroku maintenance mode during migration if it's already enabled.
This helps when we want to enable/disable the maintenance mode manually or externally.
[sayanarijit]

* Add options to manually enable or disable Heroku maintenance mode.
Use `pyramid_heroku.maintenance` script to manage the maintenance state.
[sayanarijit]

* Heroku API client related code has been moved from `pyramid_heroku.migrate` to
`pyramid_heroku.heroku`, while the `shell` function is now in `pyramid_heroku`.
[sayanarijit]

-----

* Add `pyramid_heroku.Host` tween to support AutoIdle add-on. Refs:
https://github.com/teamniteo/pyramid-realworld-example-app/issues/127
[zupo]

-----

* Update how alembic should be called in migrate.py. This is to reflect the
latest changes to pyramid_deferred_sqla:
https://github.com/teamniteo/pyramid_deferred_sqla/commit/b963702cab3934116fb00b6ef186959bc1627026
[zupo]

-----

* Prefer empty string than None. `{'VAL': '${VAR}'}` will expand into
`{'VAL': ''}` instead of `{'VAL': None}` if the value of `VAR` is not exported or
is an empty string.
[sayanarijit]

* To enforce that `$VAR` must be set, `${VAR:?custom error message}` syntax can be used.
[sayanarijit]