Backport36 doc fixes: PR#68 and PR#124 (#125)

* 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)

Doc/Makefile

@@ -4,7 +4,7 @@
 #
 
 # You can set these variables from the command line.
-PYTHON       = python
+PYTHON       = python3
 SPHINXBUILD  = sphinx-build
 PAPER        =
 SOURCES      =

Doc/faq/windows.rst

@@ -300,9 +300,9 @@ this respect, and is easily configured to use spaces: Take :menuselection:`Tools
 --> Options --> Tabs`, and for file type "Default" set "Tab size" and "Indent
 size" to 4, and select the "Insert spaces" radio button.
 
-Python raises :exc:`IndentationError` or :exc:`TabError` if mixed tabs 
+Python raises :exc:`IndentationError` or :exc:`TabError` if mixed tabs
 and spaces are causing problems in leading whitespace.
-You may also run the :mod:`tabnanny` module to check a directory tree 
+You may also run the :mod:`tabnanny` module to check a directory tree
 in batch mode.