py-flask-caching: updated to 1.8.0

Version 1.8.0
- **BREAKING:** Removed support for Python 2. Python 3.5 and upwards are
  supported as of now.
- Add option to specify if ``None`` is a cached value or not.
- Allow to use ``__caching_id__`` rather than ``__repr__`` as an object
  caching key.
- The RedisCache backend now support generating the key_prefix via a callable.
- Emit a warning if the ``CACHE_TYPE`` is set to ``filesystem`` but no
  ``CACHE_DIR`` is set.
- Fixes Google App Engine Memcache backend.
- Various documentation updates and fixes.

(adam)