Bump Facebook Graph API version to version 8.0
Bump Facebook Graph API version to version 8.0

This is currently the latest version of the API available.

Each API version is available for 2 years, and the previous version 3.2 API is
reaching end of life on May 4th, 2021. It would be good to upgrade to a newer
version before then in order to have time to find any version incompatibilities
before the current version is deprecated, although I haven't been able to find
any breaking changes by looking at the version history between the versions:
https://developers.facebook.com/docs/graph-api/changelog

python-social-auth/social-core,python-social-auth/social-core