Each cache entry is ~1 GB of our allotted 10 GB for the repository which
is quite a bit. We're probably losing cache entries all the time since
we add an entry per commit per pull request.
Saving the cache takes ~3 minutes
([example](2035812496)),
it's probably just slowing down CI. It's ~25% of our test runtime and
~50% of our clippy runtime.
Filter out source dists and wheels whose `requires-python` from the
simple api is incompatible with the current python version.
This change showed an important problem: When we use a fake python
version for resolving, building source distributions breaks down because
we can only build with versions we actually have.
This change became surprisingly big. The tests now require python 3.7 to
be installed, but changing that would mean an even bigger change.
Fixes#388
