Previously, mkapiref.py required python2.6, as it used format strings,
but continued to use print as a keyword, not a function. But, since 2.6
is implicitly made a requirement, we can also count on print being
available as a function.

This change adds python2.6 as a soft requirement during ./configure,
and converts all print keywords to statements. There's no need to do
anything about the actual building of the doc since sphinx-build can
run under python2 and python3.

The net result is that it no longer matters whether 'python' points to
python2 or python3 (see PEP394), because both will be able to run
mkapiref.py successfully.