bpo-16039: CVE-2013-1752: Limit imaplib.IMAP4_SSL.readline() (GH-11120)

bpo-16039: CVE-2013-1752: Limit imaplib.IMAP4_SSL.readline() (GH-11120)

* bpo-16039: CVE-2013-1752: Change use of readline() in
  imaplib.IMAP4_SSL to limit line length. Remove IMAP4_SSL.readline()
  and IMAP4_SSL.read() to inherit safe IMAP4 implementation.
* bpo-20118: reenable test_linetoolong() of test_imaplib
  on ThreadedNetworkedTests and ThreadedNetworkedTestsSSL.
  The test now sets the _MAXLINE limit to 10 characters.