Python 3.8 no longer uses PATH for searching DLLs so we have to add them manually.
Note that unlike PATH add_dll_directory() has no defined order, so if there are two same DLLs in PATH we might get a random one.
This only makes sure that 'setup.py test' and 'pytest' continue working. If you include pygobject manually you have to call os.add_dll_directory() yourself with the location of the DLLs you ship.