fix: use python 3.7 compatible AsyncMock

Author: owenpearson

test/ably/rest/encoders_test.py

@@ -15,7 +15,7 @@
 if sys.version_info >= (3, 8):
     from unittest.mock import AsyncMock
 else:
-    from unittest.mock import AsyncMock
+    from mock import AsyncMock
 
 log = logging.getLogger(__name__)
 

test/ably/rest/restauth_test.py

@@ -19,7 +19,7 @@
 if sys.version_info >= (3, 8):
     from unittest.mock import AsyncMock
 else:
-    from unittest.mock import AsyncMock
+    from mock import AsyncMock
 
 log = logging.getLogger(__name__)