bpo-40204: Pin Sphinx version to 2.3.1 in ``Doc/Makefile``. (GH-21141) (GH-21147)

diff --git a/Doc/Makefile b/Doc/Makefile
index 2169f1a..b8ca1ed 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==1.8.2 blurb python-docs-theme
+	$(VENVDIR)/bin/python3 -m pip install -U Sphinx==2.3.1 blurb python-docs-theme
 	@echo "The venv has been created in the $(VENVDIR) directory"
 
 dist:
diff --git a/Misc/NEWS.d/next/Build/2020-06-25-06-59-13.bpo-40204.GpD04D.rst b/Misc/NEWS.d/next/Build/2020-06-25-06-59-13.bpo-40204.GpD04D.rst
new file mode 100644
index 0000000..25a6d75
--- /dev/null
+++ b/Misc/NEWS.d/next/Build/2020-06-25-06-59-13.bpo-40204.GpD04D.rst
@@ -0,0 +1 @@
+Pin Sphinx version to 2.3.1 in ``Doc/Makefile``.