Remove unused imports.
[PTFM-8739] Semantically transform "git describe --always" output to semver
Port test_python target from Make to Python.
Allow specifying specific test cases to run.
Remove dead code.
Add test for output of "dx run <applet> -h".
Tweak semver version rewriting.

Previously git describe = "v0.61.0-19-gabcdef1" would get written as
"v0.61.0+19-gabcdef1"; this rewrites them to "v0.61.0+19.git.abcdef1" so
the usual tokenization by "." is more useful.