[BUG]: visibility attribute gcc on windows (mingw64)

Problem description

Building nanobind_example with gcc on Windows (mingw64) gives many warnings of the form (ci example)

warning: visibility attribute not supported in this configuration; ignored [-Wattributes]

As well as these warnings, a real project of mine also gives an import error at runtime:

ImportError: dynamic module does not define module export function (PyInit_sme)

Proposed solution

use dllimport / dllexport for visibility on windows (as was previously the case)
- as documented here: https://gcc.gnu.org/wiki/Visibility#How_to_use_the_new_C.2B-.2B-_visibility_support
- this reverts the change made in #440
don't set the visibility("hidden") attribute for namespaces on windows
- this is not supported on windows
- similar issue here: https://github.com/fmtlib/fmt/issues/1743
- this resolves the original mingw64 compilation error motivating #440

This is done in #555 which fixes both the compilation warnings seen in nanobind_example (ci example) as well as my ImportError issue in a real project.

Apologies for taking your time for a compiler you explicitly don't support - but hopefully this change is small and generic enough to be worth making. Many thanks for sharing this library!

Reproducible example code

No response

wjakob / nanobind