Switch from pybind11 to nanobind

[cqc-alec]

Closes #1800 .

It's not quite as bad as the diff size suggests: most of the changes are quite simple; the stub files are autogenerated; and stubgen.py is taken from the nanobind repo (hopefully temporarily, see below).

Wheels build tested here.

The nix build failure can be ignored.

There are a few follow-on tasks:

• Try to eliminate the leaks that can apparently occur when Python exits because the garbage collector can't do its job properly. For the moment I've just repressed the warnings. (See https://nanobind.readthedocs.io/en/latest/refleaks.html .)
• Make one or more PRs to the nanobind project with the customizations I've made to stubgen.py, so that we can actually use nanobind.stubgen instead of a modified copy.
• Generate stable ABI pytket wheels.

[cqc-alec]

This is a straight copy of https://github.com/wjakob/nanobind/blob/master/src/stubgen.py with some adaptations which I will try to upstream so that we can then use nanobind.stubgen directly. The adaptations can be viewed here.