Match CPython's islower/isupper exactly by joshuamegnauth54 · Pull Request #7646 · RustPython/RustPython

This PR fixes a regression from my last islower/isupper patch.
Python's Bytes doesn't assume an encoding, so methods like islower
should only consider ASCII casing.

I updated islower/isupper for UTF-8 and WTF-8 to match CPython more
closely. The two functions now use the same properties as CPython and
now match CPython exactly.

I updated the unit tests to pass on Python 3.15. Unicode updates
sometimes cause properties to shift. I previously tested everything on
Python 3.14, but that lead to failures that I assumed were bugs but were
actually due to Unicode differences. For example, U+0295 is a lower case
letter in older Unicode versions but is NOT in newer versions.

One of the new tests is disabled on Python 3.14 for now because it will
fail in CI till CI is bumped to 3.15.