Drop python 3.7, 3.8

[clbarnes]

No description provided.

[clbarnes]

3.8 is not deprecated according to PSF, but to numpy, which is a community standard among scientific packages.

[clbarnes]

The remaining test failures (locally at least) are

• from navis' use of csgraph using u32 instead of u64 internally, which I guess isn't being hit in navis' tests?
• from VFB's FAFB instance docker setup having a shared memory size limit which is being hit when fetching the tag table, see https://groups.google.com/g/vfb-suport/c/4yJWTKWr2fo

[clbarnes]

The sparse graph thing is very odd, it works fine in navis, the data types seem to be the same in pymaid. If you turn the sparse adjacency matrix into a dense one and back again it works fine as well.

[clbarnes]

The navis tests are not actually testing the no-igraph case, see navis-org/navis#139