Switch to using setuptools_scm

[blag]

Switch to using setuptools_scm to handle versioning.

Context

Context is here.

[WhyNotHugo]

I'll switch to pyproject.toml before doing this.

Mostly, we can generate a file with the version via:

[build-system]
requires = ["setuptools>=45", "wheel", "setuptools_scm>=6.2"]

[tool.setuptools_scm]
write_to = "user_sessions/version.py"
version_scheme = "post-release"