build(python): use setuptools-scm

[jsstevenson]

close #16

• simplifies release workflow: rather than maintaining both a version.py file and a git tag for the release, the installed package gets its version number directly from the git tag
• one less file
• clarifies version metadata for in-progress releases because they get a giant commit hash at the end of them

outstanding issue: entails some changes to our version patterns and behavior, discuss in https://github.com/GenomicMedLab/wagner-lab-issues/discussions/28. I think this also sets us up to move towards semantic release/release please, although we probably still have some steps to get there.

[github-actions[bot]]

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