Read version information from `git` in `setup.py`
Read version information from `git` in `setup.py`

This adds `git describe --tags` parsing to `setup.py` in order to create
a more accurate `version` string for the package:
- on tagged commits, it will use the tag as version
- other commits will be displayed as a postrelease version
(according to PEP440), i.e. `0.0.6+git.r0c7f952`
- on errors with git, it will use the plain `version` string defined in
`setup.py` (as managed through bumpversion).

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