GLobal macro %__python_version expands to empty string if puthon is not
installed. This causes parse errors in sandman while doing it's pre-commit check
on specfiles containing something like:

Requires: python >= %__python_version

This macro should look like this:

%__python_version %((%__python -c 'import sys; print sys.version' 2>/dev/null ||
echo "unknown") |head -1 |cut -d' ' -f1 |cut -d. -f1,2)

This will not cause any errors in build process since python-devel package is
often BuildRequire'd in such packages.