Setuptools-scm

- fix 198 by adding the `fallback_version` option, which sets the
version to be used when everything else fails.

\* fix 303 and 283 by adding the option `git_describe_command` to
allow the user to control the way that [git describe]{.title-ref} is
called.

- fix 297 - correct the invocation in version_from_scm and deprecate
it as its exposed by accident
- fix 298 - handle git file listing on empty repositories
- fix 268 - deprecate ScmVersion.extra

- fix 295 - correctly handle self install from tarballs

- fix 292 - match leading `V` character as well

<https://www.python.org/dev/peps/pep-0440/#preceding-v-character>

- re-release of 3.0.3 after fixing the release process