[postcreate] properly set is_instance_of for bootstrap entities
Entities created before CWEType itself need the is/is_instance_of
relations created later. We were actually only inserting "is".
Closes #7656606.
[migration] re-try without print_function when compiling a script fails
As part of the python3 porting we added the __future__.print_function import to
cubicweb.migration. That started affecting all migration scripts it runs,
which may not be prepared. So if compiling a script results in SyntaxError,
try again without the print_function flag.
Closes #7935030