bpo-29521 Fix two minor documentation build warnings (#41)

bpo-29521 Fix two minor documentation build warnings (#41)

Much of bpo-29521 was fixed in parallel with commit
e7ffb99 .  This cleans up the rest.

Apply parallel change to Doc/make.bat to read
"set SPHINXOPTS=-D latex_elements.papersize="
I don't have a Windows system on which to observe the warning,
but it should be necessary.

The warning:

.../workspace/cpython_github/Doc/faq/windows.rst:303:
    WARNING: unknown option: -t

In the Windows FAQ, `How do I keep editors from inserting tabs
into my Python source?`, contained a reference to a Python -t
option. In Python 2.x, this caused Python to issue warnings
about lines with mixed spaces and tabs, but as of Python 3.6
it does nothing.

Per discussion at http://bugs.python.org/issue29387, take
their wording. Python [3] raises an IndentationError or
TabError. Tabnanny is now a module.