Python since 3.8 no longer links to the interpreter library, so
symbols like `PyExc_AttributeError` will be undefined at link time.
This causes build failures when shared libraries are linked with
`-Wl,--no-undefined`. Use `TYPE MODULE` for python extension modules,
which allows to specify distinct flags via CMAKE_MODULE_LINKER_FLAGS
vs CMAKE_SHARED_LINKER_FLAGS. (The same is already used by
`pybind11_add_module(... MODULE ...)`).
Fixes#3258.
