* drop Python 3.8
Note that this PR only makes typing related changes.
Other changes can be done once this gets merged, they
are relatively few in numbers.
* bump build python version
* add typing_extensions to lint group
`dvc commit --help` was also completely broken, which is also fixed.
This adds unit tests for completion generation for both zsh/bash, and
tests that --help works for all commands.
skshetry