Add 3.9 as supported version in the layer

After latest change, the layer works with python 3.9 runtime.
This change is to update the release_layer script so that the layer will officially support python 3.9 as runtime.
This will be effective only for the next version of the agent

Author: PapaPedro

release_layer.py

@@ -8,7 +8,7 @@
 
 # The following values are used in the documentation, so any change of them requires updates to the documentation.
 LAYER_NAME = 'AWSCodeGuruProfilerPythonAgentLambdaLayer'
-SUPPORTED_VERSIONS = ['3.6', '3.7', '3.8']
+SUPPORTED_VERSIONS = ['3.6', '3.7', '3.8', '3.9']
 EXEC_SCRIPT_FILE_NAME = 'codeguru_profiler_lambda_exec'
 
 # We should release in all the regions that lambda layer is supported, not just the ones CodeGuru Profiler Service supports.