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)

kmk/common/pins.py

@@ -0,0 +1,40 @@
+try:
+    import board
+
+    PLATFORM = 'CircuitPython'
+    PIN_SOURCE = board
+except ImportError:
+    import machine
+
+    PLATFORM = 'MicroPython'
+    PIN_SOURCE = machine.Pin.board
+except ImportError:
+    from kmk.common.types import Passthrough
+
+    PLATFORM = 'Unit Testing'
+    PIN_SOURCE = Passthrough()
+
+
+def get_pin(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)
+    '''
+    return getattr(PIN_SOURCE, pin)
+
+
+class PinLookup:
+    def __getattr__(self, attr):
+        return get_pin(attr)
+
+
+Pin = PinLookup()

kmk/common/types.py

@@ -8,3 +8,19 @@ class AttrDict(dict):
     '''
     def __getattr__(self, key):
         return self[key]
+
+
+class Anything:
+    '''
+    A stub class which will repr as a provided name
+    '''
+    def __init__(self, name):
+        self.name = name
+
+    def __repr__(self):
+        return 'Anything<{}>'.format(self.name)
+
+
+class Passthrough:
+    def __getattr__(self, attr):
+        return Anything(attr)