The previous automatic versioning script was flawed. It produced the
correct package version for documentation builds and building PyPI
distributions, but produced an incorrect version when you run setup.py
from the downloaded package. Unfortunately, Python environment managers
(e.g. Pipenv, conda) resolve package version by evaluating setup.py,
not using the PyPI version.

This PR makes version generation simpler by reading the version string
from a simple file. Automatic versioning from tags is handled by
clobbering the version file from within the CI, rather than looking for
a CI environment variable on every usage.