Add `aws_lambda_handler` decorator to Client #774

Job	Run time
test (3.7, ubuntu-latest)	7m 36s
test (3.8, ubuntu-latest)	5m 55s
test (3.9, ubuntu-latest)	6m 13s
test (3.10, ubuntu-latest)	5m 35s
test (3.11, ubuntu-latest)	3m 5s
test (3.5, ubuntu-20.04)	6m 41s
test (3.6, ubuntu-20.04)	7m 45s
Finish Coveralls processing	17s
	43m 7s

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