Merge pull request #77 from tekktrik/fix/literal-import-failure

Fix compatibility for Python 3.7

Author: FoamyGuy

adafruit_rfm9x.py

@@ -28,11 +28,16 @@
     pass
 
 try:
-    from typing import Optional, Type, Literal
+    from typing import Optional, Type
     from digitalio import DigitalInOut
     from busio import SPI
     from circuitpython_typing import WriteableBuffer, ReadableBuffer
 
+    try:
+        from typing import Literal
+    except ImportError:
+        from typing_extensions import Literal
+
 except ImportError:
     pass