testsuite: Fix introspection test on Windows

Hi,

From the commit message, to fix running the introspection tests when Python 3.8.x or later is being used:

...when we are using Python 3.8.x or later. Python 3.8.x or later on Windows require one to call os.add_dll_directory() on every directory that contains dependent non-system DLLs of a module that are not bundled/installed with the module.

Since we are very likely running programs that rely on dependent items in %PATH%, make things easier for people by calling os.add_dll_directory() on all the valid paths in %PATH% in api.py, so that the test will run successfully on Windows with Python 3.8.x or later.

With blessings, thank you!