Move all python shebangs from /usr/bin/env to python3.

Move all python shebangs from /usr/bin/env to python3.

The change is only relevant to development as all the call to python scripts is done
calling C++ os::python that invoques the appropriate python version.

The change is two fold, on one hand remove all the uses of /usr/bin/env for python.

On the other hand rename all the calls to python from python to python3 making it explicit
and being compliant with PEP 394 -- The "python" Command on Unix-Like Systems:
https://www.python.org/dev/peps/pep-0394/

Remove the sheebang from src/graphics/GraphicsConverter.cpp because it is not necessary.

Some small whitespace changes.