Use TLSv1.2 for fake servers in tests (#8208)

Some Linux distros have begun disabling TLSv1.0 and TLSv1.1 by default
for security reasons, for example in Fedora 33 onwards:

https://fedoraproject.org/wiki/Changes/StrongCryptoSettings2

Use TLSv1.2 for the fake TLS servers created in the test suite, to avoid
failures due to OpenSSL disallowing TLSv1.0:

    <twisted.python.failure.Failure OpenSSL.SSL.Error: [('SSL routines',
    'ssl_choose_client_version', 'unsupported protocol')]>

Signed-off-by: Dan Callaghan <djc@djc.id.au>

changelog.d/8208.misc

@@ -0,0 +1 @@
+Fix tests on distros which disable TLSv1.0. Contributed by @danc86.

tests/http/__init__.py

@@ -145,7 +145,7 @@ class TestServerTLSConnectionFactory:
         self._cert_file = create_test_cert_file(sanlist)
 
     def serverConnectionForTLS(self, tlsProtocol):
-        ctx = SSL.Context(SSL.TLSv1_METHOD)
+        ctx = SSL.Context(SSL.SSLv23_METHOD)
         ctx.use_certificate_file(self._cert_file)
         ctx.use_privatekey_file(get_test_key_file())
         return Connection(ctx, None)