Cache dependencies in CI tests

[karlhigley]

Is your feature request related to a problem? Please describe.
Running the tests and tutorials currently takes around 15 minutes, of which 4 minutes or so (~25%) is spent installing dependencies.

Describe the solution you'd like
Use GH Cache Action to cache the dependencies and speed up the build.

Describe alternatives you've considered

• Leave it as is. Slower, but works.
• Speed up the tests and tutorials. Maybe possible, but they will inevitably get slower again over time.

Additional context
We'll probably want to hash all of the pip-dep/*.txt files in order to come up with the cache key.

[imskr]

I would like to work on this @karlhigley

[karlhigley]

@imskr Thanks! Feel free to ping me on Slack with any questions.