Drf-extra-fields

- A typo in the `python_requires` argument of `setup.py` that prevents installation for Python 3.6 is fixed.

- :warning: **BACKWARD INCOMPATIBLE** :warning:
- Support for `Python 3.5` is ended.
- `Python 3.9` and `Python 3.10` are now supported.
- `Django 3.2` is now supported.

- Make `psycopg2` dependency optional.

- **Possible Breaking Change**:
- In this version we have changed file class used in `Base64FileField` from `ContentFile` to `SimpleUploadedFile` (you may see the change [here](https://github.com/Hipo/drf-extra-fields/pull/149/files#diff-5f77bcb61083cd9c026f6dfb3b77bf8fa824c45e620cdb7826ad713bde7b65f8L72-R85)).
- `child_attrs` property is added to [RangeFields](https://github.com/Hipo/drf-extra-fields#rangefield).

- Bug Fix for RangeFields. Issue https://github.com/Hipo/drf-extra-fields/issues/146

- add allow_empty property to range fields.