Using the MACAddressField on a model, then attempting to submit an "extra large" mac address such as `be:ef:be:ef:be:ef:be:ef` results in an IndexError, which gets bubbled out of DRF and results in a 500 ServerError.  This should raise a ValidationError instead.

Fix potential MacAddressField crash in DRF

First, unittest2 is not compatible with python 3.10. However, in Python
3.x, the standard unittest module has the functionality we used, so
unittest2 is now only used in python 2.7.

Next. changed assertItemsEqual to AssertEqual. It seems that
assertItemsEqual only compared item counts, not the instances, and was
removed due to the majority of users, this project included, assuming
that the instances were compared. See https://bugs.python.org/issue27060

environment definitions

``
RemovedInDjango41Warning: 'netfields' defines default_app_config = 'netfields.apps.NetfieldsConfig'. Django now detects this configuration automatically. You can remove default_app_config.
``

Fixes jimfunk#126