Avoid bundling tests in wheels

[stefan6419846]

cssutils currently bundles the tests inside the wheel distributions. These usually are of no additional value outside of the source distributions and thus just bloat the distribution. For this reasons, I recommend excluding them from future wheels.

This matches the official recommendation for Python packaging as well (see https://packaging.python.org/en/latest/discussions/package-formats/#what-is-a-wheel):

Wheels are meant to contain exactly what is to be installed, and nothing more. In particular, wheels should never include tests and documentation, while sdists commonly do.