Fedora 29 switched to python 3.7. It seems that there is no official
way to build a RPM package that could works with 3.6 and 3.7.
Building two different packages for each version could open the pandora
box of packaging.

With this commit an ugly hack is used: if python3.7 is found, the odoo
directory is symlinked into its site-packages dir.

fixes #30579

closes odoo/odoo#30618