Update linkcheck:

- Suppress redirects in Volto from issues to pulls
- Fix bad Jenkins link by removing obsolete section from upgrade docs

Author: stevepiercy

Makefile

@@ -189,7 +189,7 @@ linkcheck: deps  ## Run linkcheck
 
 .PHONY: linkcheckbroken
 linkcheckbroken: deps  ## Run linkcheck and show only broken links
-	cd $(DOCS_DIR) && $(SPHINXBUILD) -b linkcheck $(ALLSPHINXOPTS) $(BUILDDIR)/linkcheck | GREP_COLORS='0;31' grep -wi "broken\|redirect" --color=auto || test $$? = 1
+	cd $(DOCS_DIR) && $(SPHINXBUILD) -b linkcheck $(ALLSPHINXOPTS) $(BUILDDIR)/linkcheck | grep -wi "broken\|redirect" | GREP_COLORS='0;31' grep -vi "https://github.com/plone/volto/issues/" --color=auto || test $$? = 1
 	@echo
 	@echo "Link check complete; look for any errors in the above output " \
 		"or in $(BUILDDIR)/linkcheck/ ."

docs/backend/upgrading/version-specific-migration/upgrade-to-python3.md

@@ -305,16 +305,6 @@ These should be changed so that Python 3 is the default.
 For example, string types (or text) should be represented as `'foo'`, not `u'foo'`, and bytes types (or data) should be represented as `b'bar'`, not `'bar'`.
 Search for examples of `Py23DocChecker` in Plone's packages to find a pattern which allows updated doctests to pass in Python 2.
 
--   To test your code against `buildout.coredev`, start by browsing to [Add-ons \[Jenkins\]](https://jenkins.plone.org/view/Add-ons/).
--   Note there are jobs set up for Plone 4.3, 5.0, 5.1, and 5.2 on Python 2, and three jobs that run tests for Plone 5.2 on Python 3.6, Python 3.7, Python 3.8, and Python 3.9.
--   Click the link {guilabel}`log in` on Jenkins website (top right).
-    For the first login, you must authorize Jenkins to have access to your GitHub account to authenticate.
--   Click the link for the job you want to run.
-    For example, {guilabel}`Test add-on against Plone 5.2 on Python3.8`.
--   Choose the link {guilabel}`Build with parameters` in the menu on the left-hand side.
--   Fill the fields {guilabel}`ADDON_URL` and {guilabel}`ADDON_BRANCH` with your repository's URL and the branch name ("python3" if you followed these instructions).
--   Start the build with the {guilabel}`Build` button.
-
 
 ## 7. Update add-on information