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.
Thanks Scott for the idea to modify the document. This seems to work, but I am
not surer whether it is safe in all cases, so better warn if this is used.
lyx2lyx did not yet know about /systemlyxdir/ and set \origin to the path
where my git tree lives instead. This path is not usable except on my machine,
so better write something more usable instead.
This is a special command line switch of lyx2lyx, so it does not interfere
with normal usage. I did not try to deduce the systemlyxdir from lyx2lyx to
be on the safe side.
