bpo-35293: Travis CI uses "make venv" for the doc (GH-22307)

Doc/requirements.txt becomes the reference for packages and package
versions needed to build the Python documentation.

* Doc/Makefile now uses Doc/requirements.txt
* .travis.yml now uses "make env" of Doc/Makefile
diff --git a/Doc/Makefile b/Doc/Makefile
index c11a7ca..f653d70 100644
--- a/Doc/Makefile
+++ b/Doc/Makefile
@@ -143,7 +143,7 @@
 venv:
 	$(PYTHON) -m venv $(VENVDIR)
 	$(VENVDIR)/bin/python3 -m pip install -U pip setuptools
-	$(VENVDIR)/bin/python3 -m pip install -U Sphinx==3.2.1 blurb python-docs-theme
+	$(VENVDIR)/bin/python3 -m pip install -r requirements.txt
 	@echo "The venv has been created in the $(VENVDIR) directory"
 
 dist: