Speed up workflow with pip cache (#394)
Speed up workflow with pip cache (#394)

Installing pip dependencies constitutes about half of our workflow
runtime, so using the builtin pip cache from the `setup-python` action
may make our CI faster. The workflows will need to be monitored to
assess the effectiveness of the cache.

GitHub will purge the cache if it goes unused for a certain amount of
time (presently 7 days).
MechanicalSoup/MechanicalSoup