Caching pip dependencies on Github Actions

[staticdev]

Installing dependencies every time is costly and could reduce workflow times if some sort of caching is used.

It takes load off PyPI, and it may make a difference to some projects. This used to be very verbose, but recently pip has gained pip cache dir. I think I also heard about a GH action for this (not sure). So feel free to open a separate issue and/or a pull request.

See https://github.com/actions/cache/blob/master/examples.md#using-pip-to-get-cache-location