[LyX/master] Move all python shebangs from /usr/bin/env to python3.

[Pavel Sanda]

> commit 3f03f0a447e0f672025d78881d8a2790133ff47b
> Author: JosĂŠ Matos <jamatos at lyx.org>
> Date:   Sat Dec 29 19:14:41 2018 +0000
>
>    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.

Jose, you might want to double check the script if you use it elsewhere, because it breaks
the whitespace in strings, see 0c8cf33ed6b.

Pavel