Fix compilation warnings

[Rhaedonius]

Just a couple of very minor tweaks to address warnings during builds:

• cython now requires nogil at the end of the declaration
• numpy requires to add #define NPY_NO_DEPRECATED_API NPY_1_7_API_VERSION to ensure we're not building against deprecated functions and fail in case they are used
• test_suite and test_requires have been deprecated for a while and have been removed from setup.py
• 'License :: OSI Approved' tag is also deprecated in PEP 639 in favour of "license-expression"
• Adding build-backend to pyproject.toml to complete the specifications for a build using PEP 518
• Force numpy>=2 during build since the resulting wheel will be compatible also with numpy<2