build: Use python3 for the post install script
While some systems alias python to python3 nowdays, this is not true for everything. Especially systems that can potentially offer both python2 and python3.
According to both PEP 394 and PEP 441 its recommended to always add the 3 in the shebang.
(This has been causing a build failure due to the the lack of /usr/bin/env python
in gnome-build-meta environment)
Edited by Jordan Petridis