support numpy2.0, require `numpy>=1.23.5`, add CI test

[tlambert03]

closes #113

note that https://github.com/scipy/oldest-supported-numpy is now deprecated

numpy has better docs and general support for depending on numpy at build time ... and this follows the advice there, namely:

• builds with "numpy>=2.0.0b1; python_version>='3.9'",
• dependencies = ["numpy>=1.23.5"]
• for python 3.8, we use the current pinnings

[tlambert03]

ready for review @marktsuchida

[marktsuchida]

@tlambert03 Thanks so much for this! Sorry for letting it sit for so long.