Bump to Python 3.8

Closes #573 (closed), closes #899 (closed)

This will require a base image build (git tag base-python-3.8) to work.

It seems #573 (closed) was caused by some changes in Python's implementation of list(…) and collections.abc.Sequence.__getitem__, which causes multiple calls to BulkMap.__len__ to be made. I could not understand a single thing from CPython's git repo, so I just chose to cache the queryset's length as an attribute of the BulkMap.