Test against numpy nightly wheels

[ianthomas23]

This adds a CI run using numpy nightly wheels as described at https://scientific-python.org/specs/spec-0004. It will therefore test against the latest numpy 2.0 build.

Not using matplotlib nightly wheels as they pin to numpy < 2.0 and only really need to test against install dependencies rather than test dependencies.

[ianthomas23]

Confirmation that this works as intended:

Downloading https://pypi.anaconda.org/scientific-python-nightly-wheels/simple/numpy/2.0.0.dev0/numpy-2.0.0.dev0-cp312-cp312-manylinux_2_17_x86_64.manylinux2014_x86_64.whl (18.0 MB)
...
Run python -m pip list
Package         Version
--------------- ----------
contourpy       1.1.2.dev1
coverage        7.3.1
cycler          0.11.0
fonttools       4.42.1
iniconfig       2.0.0
kiwisolver      1.4.5
matplotlib      3.8.0
numpy           2.0.0.dev0
packaging       23.1
Pillow          10.0.1
pip             23.2.1
pluggy          1.3.0
pyparsing       3.1.1
pytest          7.4.2
pytest-cov      4.1.0
python-dateutil 2.8.2
six             1.[16](https://github.com/contourpy/contourpy/actions/runs/6207431127/job/16852647827#step:13:17).0
wurlitzer       3.0.3