Disable the use of session tickets on TLSv1.2 by default

Since currently session resumption is not supported by urllib3
there is no reason to request tickets from the server.
It takes up extra bytes in transit (~200 bytes), and raises some
minor security concerns.

See: https://blog.filippo.io/we-need-to-talk-about-session-tickets

src/urllib3/util/ssl_.py

@@ -64,6 +64,12 @@ def _const_compare_digest_backport(a, b):
     OP_NO_COMPRESSION = 0x20000
 
 
+try:  # OP_NO_TICKET was added in Python 3.6
+    from ssl import OP_NO_TICKET
+except ImportError:
+    OP_NO_TICKET = 0x4000
+
+
 # A secure default.
 # Sources for more information on TLS ciphers:
 #
@@ -250,7 +256,7 @@ def create_urllib3_context(
         ``ssl.CERT_REQUIRED``.
     :param options:
         Specific OpenSSL options. These default to ``ssl.OP_NO_SSLv2``,
-        ``ssl.OP_NO_SSLv3``, ``ssl.OP_NO_COMPRESSION``.
+        ``ssl.OP_NO_SSLv3``, ``ssl.OP_NO_COMPRESSION``, and ``ssl.OP_NO_TICKET``.
     :param ciphers:
         Which cipher suites to allow the server to select.
     :returns:
@@ -273,6 +279,11 @@ def create_urllib3_context(
         # Disable compression to prevent CRIME attacks for OpenSSL 1.0+
         # (issue #309)
         options |= OP_NO_COMPRESSION
+        # TLSv1.2 only. Unless set explicitly, do not request tickets.
+        # This may save some bandwidth on wire, and although the ticket is encrypted,
+        # there is a risk associated with it being on wire,
+        # if the server is not rotating its ticketing keys properly.
+        options |= OP_NO_TICKET
 
     context.options |= options