bpo-39503: CVE-2020-8492: Fix AbstractBasicAuthHandler (GH-18284) (GH-19296)

bpo-39503: CVE-2020-8492: Fix AbstractBasicAuthHandler (GH-18284) (GH-19296)

The AbstractBasicAuthHandler class of the urllib.request module uses
an inefficient regular expression which can be exploited by an
attacker to cause a denial of service. Fix the regex to prevent the
catastrophic backtracking. Vulnerability reported by Ben Caller
and Matt Schwager.

AbstractBasicAuthHandler of urllib.request now parses all
WWW-Authenticate HTTP headers and accepts multiple challenges per
header: use the realm of the first Basic challenge.

Co-Authored-By: Serhiy Storchaka <storchaka@gmail.com>
Co-authored-by: Victor Stinner <vstinner@python.org>
(cherry picked from commit 0b297d4ff1c0e4480ad33acae793fbaf4bf015b4)