Exclude `tests` from the distributed wheel

[SandaruKasa]

From what I can tell, it is discouraged to have a top-level directory named tests distributed alongside with the package: https://wiki.archlinux.org/title/Python_package_guidelines#Test_directory_in_site-package So this PR excludes it from the final .whl being built.

To the best of my understanding, it broke neither the deprecated python3 setup.py test, not the tox, or pytest. However, it might have broken someone's code if it relied on tgcrypto being distributed alongside with the tests. In which case, WTF.

Anyways, this most likely needs a version bump of some sorts, no matter the breakingness of the change.