Auto-generate type stubs (#2384)

So far, we have manually generated stubs by installing
`pybind11-stubgen`, using it, and finally copying over the result
into `python/stubs`. This is tedious and tends to get forgotten, thus
stubs are outdated, and clashes with `black`
constantly.

Thus, we
- remove the static stub directory
- generate stubs directly during build time
- enable users to opt out

PS: There are some fixes to warnings thrown during the stub
construction, too.