PyPy support

PyPy 7.3.10 and newer versions are supported, though with some limitations:

1. When nanobind types occur in reference cycles, then those cycles are not collectable and will be leaked. This is a limitation of PyPy’s cpyext layer that was reported in PyPy issue #3849. Note that this would only affects users that use the nb::type_slots() feature to implement a custom Py_tp_traverse or Py_tp_clear slot.

2. nanobind normally complains about any reference leaks involving instances, functions, and types when the interpreter shuts down. PyPy lacks the final garbage collection step that is needed to identify such leaks See PyPy issue #3855. Those those checks therefore had to be disabled when compiling for PyPy.