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>

In spite of its name, the OpenSSL constant SSLv23_METHOD actually means
"use any mutually acceptable version". This avoids specifying any
particular TLS version at all.

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