Stop assuming that /usr/bin/python exists

This issue is relevant to the following ticket: legacy/trac#29913 (moved)

In short, the problem in legacy/trac#29913 (moved) was that Python 3 scripts cannot depend on a hardcoded path in order to find the Python 3 executable, because the path itself is not immutable. (For instance, as seen in that ticket itself.

Unfortunately, the same exact issue also applies to Python 2 scripts and their relevant executables.

Trac:
Username: alwayslivid