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.