[1.5.x] Fixed #20008 -- Removed trailing slash in Wikipedia link

Thanks senden9 at gmail.com for the report.
Backport of 63c52dcbcd from master.
[1.5.x] Fixed #19965 - Added a warning that the tutorial is written for Python 2.

Thanks itsallvoodoo for the patch.

Backport of e1bafdbffa from master.