`dateutil.parser` was an optional dependency for CVS tests. It was failing on macOS
beacuse the dateutil types were not being installed, and mypy was failing *even when the
CVS tests were skipped*. This seems like it was an oversight on macOS --
`types-dateutil-parser` was not installed there, though it was on Linux unit tests.
It takes 6 lines of YAML and some weird test-skipping logic to get `python-dateutil` and
`types-python-dateutil` installed in all the tests where we need them, but it only takes
4 lines of code to write the date parser we need for CVS, so I just did that instead.
Note that CVS date format can vary from system to system, but it seems like it's always
pretty similar for the parts we care about.
- [x] Replace dateutil.parser with a simpler date regex
- [x] Lose the dependency on `dateutil.parser`
a22686279c