CVE-2021-32837: mechanize Regular Expression Denial of Service vulnerability

The vulnerability stemmed from an inefficient regex pattern in AbstractBasicAuthHandler's authentication header parsing. The commit dd05334 explicitly replaced this regex with a more efficient version from Python 3.9's urllib, and added tests verifying rejection of malicious inputs like ',,,,,,...'. The regex's structure (particularly the '(?:.,)' portion) made it susceptible to ReDoS when processing headers with many unprocessed comma separators, as demonstrated in the test case.