[#3] tweak compatibility to iRODS 4.3.3

[d-w-moore]

Update the client library's advertised server compatibility to 4.3.3 in preparation for that release.

That this did not happen for release v2.1.0 will not be not an upgrade blocker for most. It affects only the generation of iRODSSession objects by the use of irods.test.helpers.make_session, because this call would fail when run with the default parameter value of test_server_version = True and connecting to an iRODS 4.3.3 server. To any users for whom this presents a problem, please import and use irods.helpers.make_session instead.