DeprecationWarning for `force_text()` in fields.py

[kccrouch]

This DeprecationWarning has started showing up in Django when using EncryptedCharField

/usr/local/lib/python3.6/dist-packages/fernet_fields/fields.py:77: RemovedInDjango40Warning: force_text() is deprecated in favor of force_str().
    return self.to_python(force_text(self.fernet.decrypt(value)))