Async-firebase

* Clean up. Remove dependencies and code that is no longer used.
* Update dependencies:
* Removed backports.entry-points-selectable (1.1.1)
* Removed chardet (4.0.0)
* Removed requests (2.25.1)
* Removed urllib3 (1.26.7)
* Updated idna (2.10 -> 3.3)
* Updated pyparsing (3.0.6 -> 3.0.7)
* Updated attrs (21.2.0 -> 21.4.0)
* Updated charset-normalizer (2.0.9 -> 2.0.12)
* Updated filelock (3.4.0 -> 3.4.1)
* Updated click (8.0.3 -> 8.0.4)
* Updated freezegun (1.1.0 -> 1.2.0)
* Updated identify (2.4.0 -> 2.4.4)
* Updated regex (2021.11.10 -> 2022.3.2)
* Installed tomli (1.2.3)
* Updated typed-ast (1.4.3 -> 1.5.2)
* Updated typing-extensions (4.0.1 -> 4.1.1)
* Updated virtualenv (20.10.0 -> 20.13.3)
* Updated google-auth (2.1.0 -> 2.6.0)
* Updated mypy (0.910 -> 0.931)

* Make _get_access_token async

* Add support of Python 3.10
* Fix typos in README.md

* Update dependencies

* Added channel_id option to Android config ([spec](https://firebase.google.com/docs/reference/fcm/rest/v1/projects.messages#AndroidNotification))

* [FIX] Adjust APS encoder in order to properly construct background push notification.