Update setup.cfg license classifier

[stes]

Changed the license classifier to License :: OSI Approved :: Apache Software License which is the closest to Apache 2.0 in the current list (https://pypi.org/pypi?%3Aaction=list_classifiers).

There is an open proposal (pypa/trove-classifiers#17) to make the classifiers more explicit, but this is still open as of today, we can update at some point in the future.

I added the "rc1" tag to this PR to be able to test the classifier in test pypi.

[stes]

looks like this (https://test.pypi.org/project/cebra/0.4.0rc1/)

[stes]

and further down it is also properly linked:

[stes]

In this PR, we could also think about bumping the development stage from Development Status :: 3 - Alpha to Development Status :: 4 - Beta -- thoughts?

Here are all stages (5 would be 1.0.0, I think):

Development Status :: 1 - Planning
Development Status :: 2 - Pre-Alpha
Development Status :: 3 - Alpha
Development Status :: 4 - Beta
Development Status :: 5 - Production/Stable
Development Status :: 6 - Mature
Development Status :: 7 - Inactive