makefile: use Python 3 by default when building docs as well
I copied the snippet defining `PYTHON` from the root Makefile.
Differential Revision: https://phab.mercurial-scm.org/D10251
@echo off
rem Windows Driver script for Mercurial
setlocal
set HG=%~f0
rem Use a full path to Python (relative to this script) if it exists,
rem as the standard Python install does not put python.exe on the PATH...
rem Otherwise, expect that python.exe can be found on the PATH.
rem %~dp0 is the directory of this script
if exist "%~dp0..\python.exe" (
"%~dp0..\python" "%~dp0hg" %*
) else (
python "%~dp0hg" %*
)
endlocal
exit /b %ERRORLEVEL%