MAINT: update handling of license files to use PEP 639

[rgommers]

Draft because it requires a future version of meson-python (xref mesonbuild/meson-python#681).

Changes:

• Use an SPDX license expression - the AND for MIT and BSD-3 is a little annoying (MIT-only is cleaner) but it is the correct thing to do, given that we have a minor amount of BSD-3 code taken over from NumPy/SciPy which wasn't written by me and hence would be annoying to relicense under the PyWavelets license.
• Remove end date of copyright statement in main license file, in order to avoid having to update this again in the future.
• Include the full NumPy and SciPy license files, as the BSD license requires.
• Remove the PyPI classifier for the license, since PEP 639 deprecated such classifiers.

[agriyakhetarpal]

Now that the licensing metadata won't be lost at the time of upload thanks to the release of https://github.com/pypa/gh-action-pypi-publish/releases/tag/v1.12.4 and Twine 6.1, this PR should be ready to go. Using the action might also make sense now, even if we don't switch to trusted publishing right away (but we should, at some point). Gentle ping, @rgommers!

[rgommers]

This still needs a meson-python 0.18.0 release with support for PEP 639 before this can be merged.

[agriyakhetarpal]

Ah, right – I missed that. I shall remove it from the 1.9.0 milestone, then; unless we want it to be included.