Adjust expected dependencies and more accurate license classifers

[Zeitsperre]

Fixes #2

Changes

• In lieu of using requires (from distutils), package dependencies use install_requires (from setuptools) to specify install-time dependencies.
• License string for GPLv3 License now conforms to Python metadata expectations.
• Other relevant metadata classifiers are now listed in the package.

Justification

What was happening previously was that because requires was present, install_requires was ignored. As such, pybind11 was not being installed by default when installing via pip. requires does not allow for non-alphanumeric characters in dependency entries (i.e.: no setting of versions).

[yrobink]

Dear @Zeitsperre,

Thanks for your PR, it is now merged