python3Packages.mautrix: 0.3.11 -> 0.4.0

Previously known as `mautrix-appservice`. For backwards-compatibility, I
added an alias in `python-packages.nix`.

Additionally a small patch was needed which I already filed upstream[1].

Please note that the package switched from a MIT license to the Mozilla
Public License[2].

[1] https://github.com/tulir/mautrix-python/pull/13
[2] https://github.com/tulir/mautrix-python/blob/v0.4.0/LICENSE