avatar: update avatar script

* Check for `python3` and `pip` to exists.
* Format and lint are now also applied to external sources.
* Fixed `Gatt` experimental servicer lint & format issues.
* Updated `mypy` to 1.1.1.

Test: avatar run
Change-Id: I06fce4fc0afd8f6991aa1ac84726a8a81c3542d2