making package requirement `psycopg2-binary` optional

[leo-schick]

I think the packages required for PostgreSQL psycopg2-binary>=2.7.3 should not be a default requirement for this package. I currently experiemnt with using BigQuery as a database backend for mara and there I do not need a PostgreSQL db connection.

To do this I suggest the following changes:

1. In version 4.8.0 we add an extra postgres with the packages required for postgres. People can then start adding it as an extra requirement in their projects.
2. In the next major version 5 we drop the packages as default requirement