Abstract away platform-dependent pin references

Quote taken straight from the docstring of get_pin:

>Cross-platform method to find a pin by string.
>
>The pin definitions are platform-dependent, but this provides
>a way to say "I'm using pin D20" without rolling a D20 and
>having to actually learn MicroPython/CircuitPython and the
>differences in how they handle pinouts.
>
>This also makes the keymap sanity checker actually work for
>CircuitPython boards, since it's not possible in CPY to
>define a module stub for `board` that uses Passthrough
>natively (which is how the MicroPython stub worked originally)

user_keymaps/klardotsh/itsybitsy_m4_express/threethree.py

@@ -1,14 +1,13 @@
-import board
-
 from kmk.common.consts import DiodeOrientation, UnicodeModes
 from kmk.common.keycodes import KC
 from kmk.common.macros.simple import send_string, simple_key_sequence
 from kmk.common.macros.unicode import unicode_sequence
+from kmk.common.pins import Pin as P
 from kmk.entrypoints.handwire.itsybitsy_m4_express import main
 from kmk.firmware import Firmware
 
-cols = (board.A4, board.A5, board.D13)
-rows = (board.D12, board.D11, board.D10)
+cols = (P.A4, P.A5, P.D13)
+rows = (P.D12, P.D11, P.D10)
 
 diode_orientation = DiodeOrientation.COLUMNS
 unicode_mode = UnicodeModes.LINUX