Hi all,
On Ubuntu Cinnamon Remix, with python3, and tkinter,

canvas.bind("<1>", callback)
canvas.bind("<2>", callback)
canvas.bind("<3>", callback)
canvas.bind("<4>", callback)
canvas.bind("<5>", callback)

are now mapped to mouse buttons, instead of keyboard number buttons.

On which platform and Python version they were mapped to keyboard number buttons?

Sequences 'a' and '1' are unambiguously interpreted as abbreviating '<Keypress-a>' and '<Keypress-1>', and similarly for any other printable ascii char.  '<a>' is interpreted also as '<Keypress-a>', but there is no reason to add the brackets.  In any case, binding a specific printable key is fairly rare.  '<1>' is ambiguous is that it could also mean '<Button-1>'.  Since '1' is available to abbreviate '<Keypress-1>', tcl currently chooses the button interpretation, and has AFAIK for at least a decade or more.  In any case, what tcl does is not a Python bug, and it does not matter what it might have done in old versions.

Note.  Buttons 4 and 5 are used on Linux for a mousewheel.  On Windows, they correspond to actual buttons, if present.  I just tested with a mouse with two side buttons.  '<6>', etc, is seen as key-6.