gh-152248: Reject a POSIX TZ abbreviation with non-ASCII-letter characters in pure-Python zoneinfo

[tonghuaroot]

The pure-Python zoneinfo parser accepts a POSIX TZ string whose unquoted std/dst abbreviation contains characters other than ASCII letters (for example an embedded space or a non-ASCII letter), while the C implementation rejects it. The unquoted alternative in the parser regex is a negated class ([^<0-9:.+-]+) that admits anything except a few delimiters, whereas the C parse_abbr walks the unquoted form with Py_ISALPHA (ASCII letters only), as POSIX (via RFC 8536) requires for the unquoted form.

This tightens the unquoted alternative to [a-zA-Z]+, matching the C accelerator and POSIX, and leaves the quoted <...> form untouched. Every well-formed TZ string and all bundled IANA zones still parse unchanged; only the previously-accepted strings now raise ValueError.

The non-ASCII case is reachable through the public from_file path, which UTF-8-decodes the footer, so it is covered by a dedicated regression test in addition to the whitespace cases added to the shared invalid_tzstrs list.

• Issue: zoneinfo: pure-Python POSIX TZ unquoted abbreviation regex accepts whitespace or non-ASCII letters (C rejects) #152248