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>
