Use meson-python

As it's not yet possible to specify Meson options in pyproject.toml,
the default is now building a Python package and developer/distro builds
should opt in to docs, testsuite and check_runtime_deps options.

Still to do:

  * ensure metadata - https://pypi.org/project/calliope-music/8.1.dev0/
    has no description (done)
  * update release process and release an 8.1 version
  * update README to document developer + test workflows (done)
  * update CI (done)
  * changelog entry (done)

Fixes: https://gitlab.com/samthursfield/calliope/-/issues/102