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.
diff --git a/Doc/Makefile b/Doc/Makefile
index 6e1c183..eb62df5 100644
--- a/Doc/Makefile
+++ b/Doc/Makefile
@@ -4,7 +4,7 @@
 #
 
 # You can set these variables from the command line.
-PYTHON       = python
+PYTHON       = python3
 SPHINXBUILD  = sphinx-build
 PAPER        =
 SOURCES      =