* Add test_namespace_visibility
To probe environment/toolchain/platform-specific behavior under the exact same conditions as normal tests.
(An earlier version of this code was used to inform PR #4043.)
* Disable flake8 in ubench/holder_comparison_*.py, to suppress new & useless diagnostics.
* Disable namespace_visibility_1s.cpp (tosee if that resolves the MSVC and CUDA `test_cross_module_exception_translator` failures).
* Turn off flake8 completely for ubench (the Strip unnecessary `# noqa`s action un-helpfully removed the added noqa).
* Disable test_namespace_visibility completely. Just keep the two .cpp files, only setting the module docstring and doing nothing else.
* Rename test_namespace_visibility.py to test_exc_namespace_visibility.py, so that it is imported by pytest before test_exceptions.py
* Add `set_property(SOURCE namespace_visibility_1s.cpp PROPERTY LANGUAGE CUDA)`
* Add reference to PR #4054
* Complete the documentation (comments in test_exc_namespace_visibility.py).
* Rename namespace_visibility.h to namespace_visibility.inl, as suggested by @charlesbeattie
