Use the local python binary to build the docs, not whatever 'python' is on the...

Use the local python binary to build the docs, not whatever 'python' is on the PATH (which is probably Python 2.x, and we need 3.x).

Doc/Makefile

@@ -73,7 +73,7 @@ TOOLSDIR=	tools
 # commontex/patchlevel.tex in their dependencies.
 RELEASE=$(shell $(PYTHON) tools/getversioninfo)
 
-PYTHON=	   python
+PYTHON=	   $(PWD)/../python
 DVIPS=	   dvips -N0 -t $(PAPER)
 
 # This is ugly!  The issue here is that there are two different levels