Turns out that even distro as new as RHEL7 does not still support
python3 properly.  This commit enables fallback to python2 on older
distros.

RPM part could be solved by rpm macros; Debian part is left to require
Python 3.4 (as per current stable); we can fix that later if need be.

Fedora 28 already issues deprecation warnings, which disrupt tests.

Shellfu v0.10.4+ will use the newer format.  This does not justify
hard dependency change, as no known functional components rely on the
version; only thing that uses it is sfdoc to display module version in
man page footer.

Already v0.1.0 since it's been used for ages as part of BMO (my other
project, not released yet.)