devel/py-graal: New port

Generic source code analyzer

Graal leverages on the Git backend of Perceval and enhances it to set up
ad-hoc source code analysis. Thus, it fetches the commits from a Git
repository and provides a mechanism to plug third party tools/libraries
focused on source code analysis.

WWW: https://github.com/chaoss/grimoirelab-graal

Sponsored by: The FreeBSD Foundation