Add Trove classifiers to the metadata

[Sviatoslav Sydorenko (Святослав Сидоренко)]

The full list of available entries is present here https://pypi.org/classifiers.

I'd probably use Development Status :: 6 - Mature, License :: OSI Approved :: BSD License, Intended Audience :: Developers, Programming Language :: Python :: 3 :: Only. Maybe add something extra if I missed anything.

[Felix Fontein]

Poetry automatically sets the license and Python classifiers BTW.