v2.1.0 - numpy v2 support; drop support for python < 3.9

This release adds support for numpy >1.26.4, including numpy >2.0

The upgrade to numpy 2.0 causes an API compatibility issue (thank you @AdamOrmondroyd for the initial fix!). Supporting numpy v1 and v2 simultaneously makes it difficult to support older versions of python; support was dropped for python versions less than 3.9 and this is now enforced as a requirement in pyproject.toml.

If python versions less than 3.9 are required, I suggest forcing installation of an older fastkde version, e.g.: pip3 install fastkde<2.1.0; the functionality of fastkde remains the same in v2.0.1 and v2.1.0.