NNGT 2.8.0: support numpy >= 2

NNGT is a python library for graph generation and analysis.

It now equires matplotlib >= 3.6 and igraph >= 0.10 (for people who use it as a backend, it stays an optional dependency).

Improvements and bugfixes

• improved colorbar for plots
• fix fallback if requested backend fails
• avoid zero width/height shapes
• improve shape fallback if shapely is missing

Compatibility

• better matplotlib compatibility for graph-tool and igraph
• move to np.nan instead of NaN for numpy 2
• move to packaging instead of pkg_resources
• move to setuptools instead of distutils
• more robust installation procedure

Maintenance and cleanup

• update build tests and args in setup.py and pyxbld
• remove use of old numpy random (randint/get_state)
• remove some unused imports
• ability to run compatibility tests for a subset of backends
• use venv for CI
• skip graph-tool in tests until it is fixed
• fix documentation