It could be something like '10beta4', which StrictVersion() would
reject. When Postgres 10 is released, it will be '10', which
StrictVersion() would STILL reject.
Fortunately, psycopg2 has a 'server_version' connection attribute that
is guaranteed to be an integer like 90605 for version 9.6.5, or 100000
for version 10. We can safely use this for version-specific code.