Fix issue #169 (force_text depracated)

[soomy]

Alias 'force_text' was deprecated in django 3.0 and is gone with 4.0
(https://docs.djangoproject.com/en/4.0/releases/3.0/#features-deprecated-in-3-0)

[agusmakmun]

@soomy nice solution, can you please use try except for that?

try:
    from django.utils.encoding import force_str  # noqa: Django>=4.x
except ImportError:
    from django.utils.encoding import force_text as force_str  # noqa: Django<=3.x

[soomy]

@agusmakmun good point, I changed the import. However the checks still fail for python<= 3.7... is this expected? Cheers & thanks for this awesome django package!