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/kdb424/handwire_planck_pyboard.py

@@ -1,14 +1,11 @@
-import machine
-
 from kmk.common.consts import DiodeOrientation, UnicodeModes
 from kmk.common.keycodes import KC
 from kmk.common.macros.unicode import unicode_sequence
+from kmk.common.pins import Pin as P
 from kmk.entrypoints.handwire.pyboard import main
 
-p = machine.Pin.board
-
-cols = (p.Y12, p.Y11, p.Y10, p.Y9, p.X8, p.X7, p.X6, p.X5, p.X4, p.X3, p.X2, p.X1)
-rows = (p.Y1, p.Y2, p.Y3, p.Y4)
+cols = (P.Y12, P.Y11, P.Y10, P.Y9, P.X8, P.X7, P.X6, P.X5, P.X4, P.X3, P.X2, P.X1)
+rows = (P.Y1, P.Y2, P.Y3, P.Y4)
 
 diode_orientation = DiodeOrientation.COLUMNS