PCRE based islower() isupper()?

[catawbasam]

PCRE seems to handle UTF-8 uppercase/lowercase correctly on Windows as well as Linux, creating a path to islower() and isupper() that works correctly across platforms.

If this seems like a reasonable approach, I'd be happy to put together a pull request.

ismatch(r"\p{Lu}", "Δ")   
true

ismatch(r"\p{Lu}", "δ")  
false

ismatch(r"\p{Ll}", "δ")  
true

ismatch(r"\p{Ll}", "Δ")   
false

[JeffBezanson]

I think this should be discussed in #5939. Would be especially interesting to benchmark.