Remove psycopg2 dependency

[PaulFarault]

Which issue(s) this PR fixes

Drop support for the psycopg2 dependency (build from sources):

• It requires some extra system dependencies that may be missing.
• It blocked the use of the --all-extras option as it does not make sense to install both postgresql and postgresql-binary at the same time.

postgresql extra dependencies has been updated to contain psycopg2-binary and postgresql-binary extra dependencies has been deleted.

This PR also adds some information about extra supported backend in the README.

Agreements

• I hereby declare this contribution to be licensed under the Apache License Version 2.0, January 2004.
• I hereby agree to grant TOSIT a copyright license to use my contributions.