Even if we don't fix all uses of (?to)(lower|upper) in the source tree,
I think it's important that codec and locale lookup work properly when
the current locale defines non-latin case folding for latin characters.
Here is a patch.

Perhaps also the str type should grow ascii_lower() and ascii_upper()
methods, since many cases of using lower() and upper() actually assume
ascii semantics (e.g. for parsing of HTTP or SMTP headers).