update `python_2_unicode_compatible` import location as part of django 3 upgrade

[nKandel]

modesl.py has python_2_unicode_compatible import from django.utils. Need to update that as a part of django 3 upgrade.

sol: replace: from django.utils.encoding import python_2_unicode_compatible line with: from six import python_2_unicode_compatible

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comment from djagno release note:
Removed private Python 2 compatibility APIs

• django.utils.encoding.python_2_unicode_compatible() - Alias of six.python_2_unicode_compatible().