Mopidy-alsamixer

- Provide full error descriptions for our MixerError exceptions. (Fixes: 26, PR: 27)

- Add Python 3 to test matrix. (PR: 29)

- Depend on final release of Mopidy 3.0.0.

- Require Mopidy >= 3.0.0a3.

- Require Python >= 3.7. No major changes required.

- Update project setup.

- Require Mopidy >= 2.0, as we from release 1.1.0 import
`gi.repository.GstAudio`, which is incompatible with `gobject` which is
used by Mopidy < 2.0.

- Don't rely on all cards which use a hardware card index to be included in the
list returned by `alsaaudio.cards()`. This can happen if an audio card is
disabled, but still use up a "card index", for example when disabling the
builtin audio card on a Raspberry Pi. (Fixes: 8)

- Add `alsamixer/min_volume` and `alsamixer/max_volume` config values to
make Mopidy-ALSAMixer's volume scale only use a part of the underlying ALSA
volume scale. (PR: 9)

- Add `alsamixer/volume_scale` to allow switching between `linear`,
`cubic`, and `log` scales. The default value has been changed from
`linear` to `cubic`, which is closer to how the human ear percieve the
volume. (Fixes: 3, PR: 9)

- Don't crash on epoll being interrupted by the machine suspending.