[docs] Emit major version based canonical URLs for docs (issue 26355)

http://bugs.python.org/review/26355/diff/19842/Doc/tools/templates/layout.ht... File Doc/tools/templates/layout.html (right): http://bugs.python.org/review/26355/diff/19842/Doc/tools/templates/layout.ht... Doc/tools/templates/layout.html:41: <link rel="canonical" href="https://docs.python.org/3/{{pagename}}.html"/> Can we add a space before '/>' to be consistent with the rest of file? http://bugs.python.org/review/26355/