docs: def match_hostname(cert, hostname):
      """
      This is a backport of the match_hostname() function from Python 3.2,
      essential when using SSL.
      Verifies that *cert* (in decoded format as returned by
      SSLSocket.getpeercert()) matches the *hostname*.  RFC 2818 rules
      are mostly followed, but IP addresses are not accepted for *hostname*.
  
      CertificateError is raised on failure. On success, the function
      returns nothing.
      """
      if not cert:
          raise ValueError("empty or no certificate")
      dnsnames = []
      san = cert.get('subjectAltName', ())
      for key, value in san:
          if key == 'DNS':
              if _dnsname_to_pat(value).match(hostname):
                  return
              dnsnames.append(value)
      if not san:
          # The subject is only checked when subjectAltName is empty
          for sub in cert.get('subject', ()):
              for key, value in sub:
                  # XXX according to RFC 2818, the most specific Common Name
                  # must be used.
                  if key == 'commonName':
                      if _dnsname_to_pat(value).match(hostname):
                          return
                      dnsnames.append(value)
      if len(dnsnames) > 1:
          raise CertificateError("hostname %r "
                                 "doesn't match either of %s"
                                 % (hostname, ', '.join(map(repr, dnsnames))))
      elif len(dnsnames) == 1:
          raise CertificateError("hostname %r "
                                 "doesn't match %r"
                                 % (hostname, dnsnames[0]))
      else:
          raise CertificateError("no appropriate commonName or "
                                 "subjectAltName fields were found")