New explicit versioning mechanism

[simonw]

• Remove all references to versioneer
• Re-implement versioning to use a static string baked into the repo
• Ensure that string is output by datasette --version and /-/versions

Refs #1054

[codecov]

Codecov Report

Merging #1060 into main will increase coverage by 6.42%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1060      +/-   ##
==========================================
+ Coverage   84.71%   91.13%   +6.42%     
==========================================
  Files          28       27       -1     
  Lines        3957     3677     -280     
==========================================
- Hits         3352     3351       -1     
+ Misses        605      326     -279     

Impacted Files	Coverage Δ
datasette/cli.py	73.63% <100.00%> (+0.13%)	⬆️
datasette/version.py	100.00% <100.00%> (ø)

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