Add level3_shift special key

[Glandos]

In a lot of keyboard layouts (See the output of grep -r ralt_switch /usr/share/X11/xkb/), the AltGr is mapped to the ISO_Level3_Shift.
I decided to strip the ISO_ prefix from the member name. Feel free to integrate it if you like.

[moses-palmer]

Thank you for your pull request.

Does this solve a bug with pynput? pynput.Keyboard.Key.alt_gr is intended to be usable as ISO_Level3_Shift.

[Glandos]

With your test program, I have the following output when pressing and releasing my AltGr key, with a French (Bepo) layout:

alphanumeric key None pressed
<65027> released

But since your last comment, I tested the following code:

from pynput.keyboard import Key, Controller
keyboard = Controller()
with keyboard.pressed(Key.alt_gr):
    keyboard.press('a')
    keyboard.release('a')

with keyboard.pressed(Key.level3_shift):
    keyboard.press('a')
    keyboard.release('a')

And I have the following output:

æa

This is not the correct output. The correct output should be ææ with my current layout. So I don't know how to make the key recognized and working with fake inputs. Any idea?

[precondition]

Suggested change

	alt_gr = KeyCode._from_symbol('Mode_switch')
	level3_shift = KeyCode._from_symbol('ISO_Level3_Shift')
	alt_gr = KeyCode._from_symbol(''ISO_Level3_Shift')
	mode_switch = KeyCode._from_symbol('Mode_switch')

Most xkb layouts in /usr/share/X11/xkb/symbols by far assign the 'ISO_Level3_Shift to the right AltGr key. Only digital_vndr/pc nec_vndr/jp pk sun_vndr/tw and sun_vndr/jp use Mode_switch. With that mind, it makes more sense to swap the two.

PS: The alphabetical order is broken with my suggestion so don't hit the "Apply" button. Move mode_switch between left and page_down to preserve the alphabetical order.

[thomaslc66]

Line 90 + double quote ''ISO_Level3_Shift'