Collect CI workflows back into one file

Splitting up the previous workflows into separate files did work, but on
balance, I find the disadvantages outweigh the advantages. The
modularity of the separate files approach was good for reducing
cognitive load and making maintenance a little bit easier, but it also
decreased overall execution efficiency because the organization I used
required more jobs.

This new single file `ci.yaml` puts all the CI checks into one place,
and adds some additional lint jobs compared to the previous set of
workflow files.

Note: this `ci.yaml` file does not have the wheel-building steps that
the previous setup had. The wheel builds and PyPI uploads will be
reintroduced in an upcoming separate workflow.

Author: mhucka