Make sdist packages usable

[chrysn]

Closes: #289

[chrysn]

I think this is ready for review; it passes CI.

Changes apart from "fixing the issue" are:

• On the platform that builds the sdist, also build the binaries from the sdist. (It'd be nice to build all from the sdist, but that'd create complex interactions, and chances are that that orthogonal niche doesn't attract bugs).
• CI runs Python tests now on every PR, helping spot similar issues early when more crates are added.

After this got merged, please consider adding the "build python wheels" check(s) to the mandatory list for merging.

[geonnave]

adding ... to mandatory list for merging

According to this, I may not have permission to do that (seems to require organisation's owner permission). cc @malishav.

[chrysn]

There's no hurry with that: having those builds visible and non-blocking would still be an improvement.