…opers#1659

When gitpython-developers#1659 was updated to pick up linting configuration changes, it
inadvertently undid one of the URL changes made in gitpython-developers#1662, putting
the URL in the git.exc module back to the one that redirects to a
different BSD license from the one this project uses.

Since only that one module was affected, the fix is simple. This
only changes the URL back; it doesn't undo any other gitpython-developers#1659 changes.