Use git to get whipper's version

[Freso]

This essentially a copy of code from spotify2musicbrainz: https://gitlab.com/Freso/spotify2musicbrainz/commit/59157165c4ca4e7bb1d5d8b081365029ddf55c72

Given that that code is in GPLv3 (and I'm its author :)), it should be fine to use. It is mostly boilerplate from upstream documentation anyway: https://pypi.org/project/setuptools-scm/

Should fix #337

[JoeLametta]

@Freso What happens if you download whipper's sources without using git? (like this)

[Freso]

(venv) (venv) freso@koume ~/t/w/whipper-task-issue-337-use-scm-version> python setup.py install --optimize=1
Traceback (most recent call last):
  File "setup.py", line 19, in <module>
    ('share/metainfo', ['com.github.whipper_team.Whipper.metainfo.xml']),
  File "/tmp/freso-tmp/whipper/venv/lib/python2.7/site-packages/setuptools/__init__.py", line 145, in setup
    return distutils.core.setup(**attrs)
  File "/usr/lib64/python2.7/distutils/core.py", line 111, in setup
    _setup_distribution = dist = klass(attrs)
  File "/tmp/freso-tmp/whipper/venv/lib/python2.7/site-packages/setuptools/dist.py", line 447, in __init__
    k: v for k, v in attrs.items()
  File "/usr/lib64/python2.7/distutils/dist.py", line 287, in __init__
    self.finalize_options()
  File "/tmp/freso-tmp/whipper/venv/lib/python2.7/site-packages/setuptools/dist.py", line 739, in finalize_options
    ep.load()(self, ep.name, value)
  File "/tmp/freso-tmp/whipper/venv/lib/python2.7/site-packages/setuptools_scm/integration.py", line 23, in version_keyword
    dist.metadata.version = get_version(**value)
  File "/tmp/freso-tmp/whipper/venv/lib/python2.7/site-packages/setuptools_scm/__init__.py", line 144, in get_version
    parsed_version = _do_parse(config)
  File "/tmp/freso-tmp/whipper/venv/lib/python2.7/site-packages/setuptools_scm/__init__.py", line 111, in _do_parse
    "use git+https://github.com/user/proj.git#egg=proj" % config.absolute_root
LookupError: setuptools-scm was unable to detect version for '/tmp/freso-tmp/whipper/whipper-task-issue-337-use-scm-version'.

Make sure you're either building from a fully intact git repository or PyPI tarballs. Most other sources (such as GitHub's tarballs, a git checkout without the .git folder) don't contain the necessary metadata and will not work.

For example, if you're using pip, instead of https://github.com/user/proj/archive/master.zip use git+https://github.com/user/proj.git#egg=proj

[JoeLametta]

Merged, thanks!