bpo-30688: Import unicodedata only when needed. (GH-5606)

bpo-30688: Import unicodedata only when needed. (GH-5606)

Importing unicodedata in sre_parse leads to failure in compilation.
unicodedata is unused during compilation, and is not compiled when this
file is imported. The error occurs when generating posix variables,
pprint is required. The dependency chain goes on like this:

sysconfig -> pprint -> re -> sre_compile -> sre_parse (this file)

This commits fixes compilation issues introduced by
2272cec13b53c405d86c45d404f035f201c0baef.
(Issue 30688, GH-5588)