I think the requirements.txt min is too low for neutron. It probably needs to target master for now since 16.0.0 has 'import mock'. I'm not sure why that is. With a fresh git clone of networking-odl and new tox env, tox -e py38 results in errors such as:

--- import errors ---                                                                                                                                                                                                                                                             
Failed to import test module: networking_odl.tests.unit.bgpvpn.test_odl_v2                                                                                                                                                                                                        
Traceback (most recent call last):                                                                                                                                                                                                                                                
  File "/usr/lib/python3.8/unittest/loader.py", line 436, in _find_test_path                                                                                                                                                                                                      
    module = self._get_module_from_name(name)                                                                                                                                                                                                                                     
  File "/usr/lib/python3.8/unittest/loader.py", line 377, in _get_module_from_name                                                                                                                                                                                                
    __import__(name)                                                                                                                                                                                                                                                              
  File "/home/corey/pkg/victoria/upstream/networking-odl/networking_odl/tests/unit/bgpvpn/test_odl_v2.py", line 23, in <module>                                                                                                                                                   
    from networking_odl.tests.unit import base_v2                                                                                                                                                                                                                                 
  File "/home/corey/pkg/victoria/upstream/networking-odl/networking_odl/tests/unit/base_v2.py", line 18, in <module>                                                                                                                                                              
    from neutron.tests.unit.plugins.ml2 import test_plugin                                                                                                                                                                                                                        
  File "/home/corey/pkg/victoria/upstream/networking-odl/.tox/py38/lib/python3.8/site-packages/neutron/tests/unit/plugins/ml2/test_plugin.py", line 21, in <module>                                                                                                               
    import mock                                                                                                                                                                                                                                                                   
ModuleNotFoundError: No module named 'mock'