www/py-social-auth-app-django: Fix build after 3496c09fda1b

• Although there are already newer releases that are compatible with py-social-auth-core >= 5, they also introduce quite a few breaking changes along with py-social-auth-core 4.9.0+, so updating the port is not an option at this time.
  
  Therefore, switch to the recently created py-social-auth-core4 port for now to fix the fallout and avoid regressions.

• Bump PORTREVISION due dependency change.

PR: 297152
Reported by: pkg-fallout