Version 2.8.0: support numpy >= 2

NNGT now equires matplotlib >= 3.6 and igraph >= 0.10.

**Improvements and bugfixes**

- improved colorbar for plots
- fix fallback if requested backend fails
- avoid zero width/height shapes
- improve shape falback 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