Friendlier behaviour when __version__ and git tag mismatch

[almarklein]

Sometimes the hardcoded __version__ and the tag reported by git mismatch. This can happen because:

• You just bumped __version__ and did not yet create a tag.
• You created a tag, but made a typo.
• You are in a long-running branch, a new release was made in main, and you merged that into the branch.
  • We use --firts-parent (which I still think makes sense), but it does mean that git will report the version from which we started the branch.

Old behavior:

• Show a warning, but this can cause tests that check logs to fail.
• We went with the git tag, but this causes problems, e.g. when building dev wheels (flit will read the version).

New behavior:

• Always go ith __version__.
• Detect that 3d case above, and don't show a warning in that case.
• Include the git-reported version info like such: '2.3.0+from_tag_2_2_1.post35.gd501006.dirty'