fix error on django>=3.0 that occurs due to a removing `python_2_unicode_compatible` from the latest versions

[Sanshain]

python_2_unicode_compatible function was removed in django 3.0. This caused the error

ImportError: cannot import name ‘python_2_unicode_compatible’ from ‘django.utils.encoding’

in models.py.

python version == 3.6 django version == 3.2 django-jet == 1.0.8

Now I set the function stub for all version python with major version==3 with a goal avoid the error