stack_decode.py: Adjust to work with Python < 3.7 (#10113)
stack_decode.py: Adjust to work with Python < 3.7 (#10113)

Specifically, the envoyproxy/envoy-build-ubuntu Docker image still has
Python 3.5.

Signed-off-by: Luke Shumaker <1abe3165bcc8c29e8b31e5c88636540fb9350004@datawire.io>
envoyproxy/envoy,eklitzke/envoy,lyft/envoy,lyft/envoy,lyft/envoy,envoyproxy/envoy,lyft/envoy,envoyproxy/envoy,lizan/envoy,istio/envoy,envoyproxy/envoy,envoyproxy/envoy-wasm,eklitzke/envoy,envoyproxy/envoy,eklitzke/envoy,envoyproxy/envoy,istio/envoy,lizan/envoy,envoyproxy/envoy-wasm,eklitzke/envoy,lyft/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy,istio/envoy,lizan/envoy,envoyproxy/envoy,eklitzke/envoy,eklitzke/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy,istio/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy,istio/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy-wasm,lyft/envoy,lizan/envoy,envoyproxy/envoy,envoyproxy/envoy,lizan/envoy,istio/envoy,lizan/envoy,istio/envoy