CVE-2021-3733: fix ReDoS in urllib2 AbstractBasicAuthHandler regex; the
legacy '(?:.,)' prefix is replaced with the upstream-3.x form
'(?:^|,)' and the scheme charset excludes ',' to prevent quadratic
backtracking on crafted WWW-Authenticate headers
CVE-2021-23336: stop accepting ';' as a default query-string separator
in urlparse.parseqs/parseqsl and cgi.parse* / FieldStorage; only '&'
is used by default, with an opt-in 'separator' kwarg for callers that
need legacy behavior
Additional tests for CVE-2021-23336: drop obsolete legacy-';' entries
from Lib/test/testcgi.py parsestricttestcases