bpo-30607: Use external python-doc-theme (GH-2017)

diff --git a/Doc/Makefile b/Doc/Makefile
index 307d1e0..042f960 100644
--- a/Doc/Makefile
+++ b/Doc/Makefile
@@ -123,7 +123,7 @@
 
 venv:
 	$(PYTHON) -m venv $(VENVDIR)
-	$(VENVDIR)/bin/python3 -m pip install -U Sphinx blurb
+	$(VENVDIR)/bin/python3 -m pip install -U Sphinx blurb python-docs-theme
 	@echo "The venv has been created in the $(VENVDIR) directory"
 
 dist: