* Travis CI: run rstlint.py in the docs job (#68)

Currently, http://buildbot.python.org/all/buildslaves/ware-docs
buildbot is only run as post-commit. For example, bpo-29521 (PR#41)
introduced two warnings, unnotified by the Travis CI docs job.

Modify the docs job to run toosl/rstlint.py.

Fix also the two minor warnings which causes the buildbot slave to
fail.
(cherry picked from commit 2b501866)

* Doc/Makefile: set PYTHON to python3 (#124)

rstlint.py run by "make check" doesn't support Python 2.

"make venv" runs "$(PYTHON) -m venv", whereas Python 2 doens't
provide the venv module: it's a module of Python 3 standard library.

(cherry picked from commit 91b0e7d0)