fallback for Literal python37

Author: FoamyGuy

adafruit_fona/adafruit_fona.py

@@ -27,10 +27,15 @@
 from simpleio import map_range
 
 try:
-    from typing import Optional, Tuple, Union, Literal
+    from typing import Optional, Tuple, Union
     from circuitpython_typing import ReadableBuffer
     from busio import UART
     from digitalio import DigitalInOut
+
+    try:
+        from typing import Literal
+    except ImportError:
+        from typing_extensions import Literal
 except ImportError:
     pass
 

adafruit_fona/fona_3g.py

@@ -24,9 +24,13 @@
 from .adafruit_fona import FONA, REPLY_OK
 
 try:
-    from typing import Optional, Tuple, Union, Literal
+    from typing import Optional, Tuple, Union
     from busio import UART
     from digitalio import DigitalInOut
+    try:
+        from typing import Literal
+    except ImportError:
+        from typing_extensions import Literal
 except ImportError:
     pass