add keypad scanner for shift registers

docs/scanners.md

@@ -74,6 +74,31 @@ class MyKeyboard(KMKKeyboard):
 ```
 
 
+### keypad ShiftRegisterKeys
+
+This scanner can read keys attached to a parallel-in serial-out shift register
+like the 74HC165 or CD4021. Note that you may chain shift registers to load in
+as many values as you need.
+```python
+from kmk.scanners.keypad import ShiftRegisterKeys
+
+class MyKeyboard(KMKKeyboard):
+    def __init__(self):
+        # create and register the scanner
+        self.matrix = ShiftRegisterKeys(
+            # require arguments:
+            clock=board.GP0,
+            data=board.GP1,
+            latch=board.GP2,
+            key_count=8,
+            # optional arguments with defaults:
+            value_to_latch=True, # 74HC165: True, CD4021: False
+            value_when_pressed=False,
+            interval=0.02,
+            max_events=64
+        )
+```
+
 ## Digitalio Scanners
 
 ### digitalio MatrixScanner

kmk/scanners/keypad.py

@@ -88,3 +88,29 @@ class KeysScanner(KeypadScanner):
             max_events=max_events,
         )
         super().__init__()
+
+
+class ShiftRegisterKeys(KeypadScanner):
+    def __init__(
+        self,
+        *,
+        clock,
+        data,
+        latch,
+        value_to_latch=True,
+        key_count,
+        value_when_pressed=False,
+        interval=0.02,
+        max_events=64,
+    ):
+        self.keypad = keypad.ShiftRegisterKeys(
+            clock=clock,
+            data=data,
+            latch=latch,
+            value_to_latch=value_to_latch,
+            key_count=key_count,
+            value_when_pressed=value_when_pressed,
+            interval=interval,
+            max_events=max_events,
+        )
+        super().__init__()