0006713: Databases written with pickle protocol 3 (Python3) should not be opened with pickle protocol 2 (Python2). Also give warning when about to upgrade a Python2 database to Python3.

git-svn-id: svn+ssh://svn.code.sf.net/p/gramps/code/trunk@22243 4ae1f11a-8b86-4847-b8af-ab372f36d1fd