.python-version always matches the minimum supported Python version.
It's main purpose is to catch accidental use of too modern syntax
in scripts and functional tests.
We (currently) don't specify a minimum patch version, so it's not
necessary to do so here. The minor verion is enough.
This also avoids requiring users to keep a potentially unsafe old
patch version installed.
Make lint/check-doc.py Python 3.4 compatible.
Also add .python-version for pyenv which will cause tests with too
modern syntax to fail on developer machine rather than on Travis.