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

[berker.peksag at gmail.com]

http://bugs.python.org/review/26355/diff/19842/Doc/tools/templates/layout.html
File Doc/tools/templates/layout.html (right):

http://bugs.python.org/review/26355/diff/19842/Doc/tools/templates/layout.html#newcode41
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/