update awscrt to 0.8.0 (#100)

-    API CHANGE: awscrt.auth.AwsSigningConfig.signed_body_value is now a string instead of an enum.
 -   API CHANGE: Added check for iOS platform
 -   BUGFIX: websocket no longer hangs if CLOSE frame cannot be sent.
  -  BUGFIX: Fixes a crash when shutting down an mqtt connection with an incomplete request with no callback from aws-c-mqtt


Co-authored-by: Dengke Tang <dengket@amazon.com>

Author: TingDaoK

setup.py

@@ -13,7 +13,7 @@
     url='https://github.com/aws/aws-iot-device-sdk-python-v2',
     packages = ['awsiot'],
     install_requires=[
-        'awscrt==0.6.2',
+        'awscrt==0.8.0',
         'futures;python_version<"3.2"',
         'typing;python_version<"3.5"',
     ],