Fix Django 6.0+ version compatibility for ChoicesType import

Simplify version check to use if/else instead of elif with upper bound.
The previous `elif django.VERSION <= (6, 0)` failed for Django 6.0 due
to tuple comparison semantics.

Fixes https://github.com/brianjbuck/drf_orjson_renderer/issues/23
Fixes https://github.com/brianjbuck/drf_orjson_renderer/issues/30