How to disable multiselection on Treeview in tkinter
The answer is a bit higher up the page:
selectmode
Controls how the built-in class bindings manage the selection. One of “extended”, “browse” or “none”. If set to “extended” (the default), multiple items may be selected. If “browse”, only a single item will be selected at a time. If “none”, the selection will not be changed.
Note that the application code and tag bindings can set the selection however they wish, regardless of the value of this option.
Setting selectmode="browse" should give the behavior you're asking for.