add a fallback_version to the setuptools_scm configuration

[keewis]

Follow-up to #4299. While investigating version issues on RTD, I noticed that setuptools_scm allows configuring its behavior, and the custom try: ... except ... clause can be replaced by setting fallback_version. The advantage is that once we switch to pyproject.toml (for which we need to drop python 3.6), the changes are much more straightforward.

cc @crusaderky

[crusaderky]

Tested, it works. Good catch.