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.

To make it a bit easier to maintain multiple Shellfu/Bash libraries, we
want to have sort of "template" and keep every module as close to it as
practical.

Replace tabs with spaces and use more logical grouping

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