Doc: Bump Sphinx verison. (python#13785)

To reflect the one we're using in production.

.azure-pipelines/docs-steps.yml

@@ -12,7 +12,7 @@ steps:
   inputs:
     versionSpec: '>=3.6'
 
-- script: python -m pip install sphinx==1.8.2 blurb python-docs-theme
+- script: python -m pip install sphinx==2.0.1 blurb python-docs-theme
   displayName: 'Install build dependencies'
 
 - ${{ if ne(parameters.latex, 'true') }}:

.travis.yml

@@ -55,7 +55,7 @@ matrix:
         # Sphinx is pinned so that new versions that introduce new warnings won't suddenly cause build failures.
         # (Updating the version is fine as long as no warnings are raised by doing so.)
         # The theme used by the docs is stored separately, so we need to install that as well.
-        - python -m pip install sphinx==1.8.2 blurb python-docs-theme
+        - python -m pip install sphinx==2.0.1 blurb python-docs-theme
       script:
         - make check suspicious html SPHINXOPTS="-q -W -j4"
     - name: "Documentation tests"

Doc/Makefile

@@ -133,7 +133,7 @@ clean:
 venv:
 	$(PYTHON) -m venv $(VENVDIR)
 	$(VENVDIR)/bin/python3 -m pip install -U pip setuptools
-	$(VENVDIR)/bin/python3 -m pip install -U Sphinx blurb python-docs-theme
+	$(VENVDIR)/bin/python3 -m pip install -U Sphinx==2.0.1 blurb python-docs-theme
 	@echo "The venv has been created in the $(VENVDIR) directory"
 
 dist: