remove dynamic metadata

[chris48s]

This PR would switch from using dynamic = ["version"] and __version__ as the source of truth to using importlib.metadata.version and pyproject.toml as the source of truth.

Some notes:

• importlib is available in python >=3.10. For older versions you have to require importlib-metadata. That isn't a problem for this library because I'm already doing that and using importlib for other things. For other libraries it would be quite annoying to pull in a whole library for the sake of a version number. I'd be inclined to only do this more widely if I'm dropping python < 3.10
• Flit doesn't have a command for bumping the version number and is unlikely to gain one. We can make a little utility for this, but python doesn't have a library that can write toml files in the standard library. In >= 3.11, there is a toml reader the standard lib but no writer. So we have to pull in a toml library to do this. It only has to be a dev dependency, but it is another package. You could avoid this by either switching to another build tool (e.g: hatch) or doing the version bump by hand.

Other places to update/account for this:

• https://github.com/chris48s/geometry-to-spatialite/
• https://github.com/chris48s/cookiecutter-python-package/
• https://github.com/chris48s/python-package-shared/blob/c21e93468cf4fa39aa99b5bea26d1ec2aabe2c38/.github/workflows/write-badges-pep621.yml#L67-L73

[socket-security]

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Package	New capabilities	Transitives	Size	Publisher
pypi/[email protected]	filesystem, shell, unsafe Transitive: environment, eval, network	+764	7.56 GB	neersighted

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