CI testing in Python 39 [2nd try]

[jGaboardi]

Current failures Python 3.9 support:

======================================================================= short test summary info =======================================================================
FAILED tigernet/tests/test_tigernet_empirical_gdf.py::TestNeworkTopologyEmpiricalGDF::test_network_node2node - AssertionError: Lists differ: [5, 8, 9] != [8, 9, 5]
FAILED tigernet/tests/test_tigernet_empirical_gdf.py::TestNeworkComponentsEmpiricalGDF::test_network_ndata_components - AssertionError: Lists differ: [360, 99, 99, ...
FAILED tigernet/tests/test_tigernet_empirical_gdf.py::TestNeworkComponentsEmpiricalGDF::test_network_node_components_largest - KeyError: 281
FAILED tigernet/tests/test_tigernet_empirical_gdf.py::TestNetworkSimplifyEmpiricalGDF::test_simplify_copy_node_cc - KeyError: 89
FAILED tigernet/tests/test_tigernet_empirical_gdf.py::TestNetworkSimplifyEmpiricalGDF::test_simplify_inplace_node_cc - KeyError: 89
FAILED tigernet/tests/test_tigernet_synthetic.py::TestNetworkSimplifyBarb::test_simplify_copy_segm_cc - AssertionError: {2: [0, 1, 2]} != {1: [0, 1, 2]}
FAILED tigernet/tests/test_tigernet_synthetic.py::TestNetworkSimplifyBarb::test_simplify_inplace_segm_cc - AssertionError: {2: [0, 1, 2]} != {1: [0, 1, 2]}

See first attempt in #37.

[jGaboardi]

Addressing in #41.