Add PyPy support

[nicholasjng]

nanobind supports PyPy bindings builds, and so should nanobind-bazel.

Currently, the CPython header dep is hardcoded, but building with PyPy instead should work similarly.

To the best of my knowledge, rules_python has no PyPy support, so users are (currently) on their own when supplying the headers. It's possible today to monkey-patch the Python building the wheel with a Bazel toolchain declaration, which could be a way forward here.

Needs investigation on my part into how to build C++ extensions for PyPy.

[nicholasjng]

Follow-up of #1.