Exclude unused platform-specific tkdnd files in pyinstaller hook

[TransparentLC]

The tkdnd folder contains pre-compiled files for Windows, Linux and macOS. But TkinterDnD.py only loads one of them according to current platform. I modified the pyinstaller hook to exclude the unused platform-specific tkdnd files so the bundled app's size can be slightly reduced.

[demberto]

Afaik this can be done for the setup.cfg also (PyPi build)

[Squiblydoo]

Sorry for the long period since your submission of this PR. Thanks for the improvement suggestion. I have reviewed this and determined that it works as expected.