Beets-check

- Don’t run custom external programs with `-v` (e.g. `ffmpeg -v`) to determine
whether they are available. (Fixes 43)
- Require Python >=3.9

- Require beets >=1.6.1 and support beets v2.x

- Require Python ^3.8
- Require beets ^1.6

- Drop support for Python2.7
- Require `beets>=1.4.7`
- Fix a crash in `beet check --add` when a music file is not found on disk. (ssssam)

- Fix crash when running `beet import` with threading enabled ([22](https://github.com/geigerzaehler/beets-check/issues/22)) ([alebianco](https://github.com/alebianco))

- Add support for Python 3
- Drop support for `beets<=1.3.10`