Write a basic keymap sanity checker utility

setup.cfg

@@ -4,6 +4,7 @@ max_line_length = 99
 ignore = X100
 per-file-ignores =
 	user_keymaps/**/*.py: F401,E501
+	tests/test_data/keymaps/**/*.py: F401,E501
 
 [isort]
-known_third_party = analogio,bitbangio,bleio,board,busio,digitalio,framebuf,gamepad,gc,microcontroller,micropython,pulseio,pyb,pydux,uio,ubluepy,machine,pyb
+known_third_party = analogio,bitbangio,bleio,board,busio,digitalio,framebuf,gamepad,gc,microcontroller,micropython,pulseio,pyb,pydux,uio,ubluepy,machine,pyb,uos