Issue #17386 make.bat must run with Python 2 until Sphinx runs with Python 3.

If PYTHON is undefined, this fails without the launcher (installed with 3.3 or
from PyPI), but this is better than always failing. Patch from Zachary Ware.

Doc/make.bat

@@ -2,7 +2,7 @@
 setlocal
 
 set SVNROOT=http://svn.python.org/projects
-if "%PYTHON%" EQU "" set PYTHON=..\pcbuild\python
+if "%PYTHON%" EQU "" set PYTHON=py -2
 if "%HTMLHELP%" EQU "" set HTMLHELP=%ProgramFiles%\HTML Help Workshop\hhc.exe
 if "%DISTVERSION%" EQU "" for /f "usebackq" %%v in (`%PYTHON% tools/sphinxext/patchlevel.py`) do set DISTVERSION=%%v