Install Python3 to wasm32 CI since wabt removes Python2 support (#840)

See WebAssembly/wabt#1321

Author: makotokato

ci/docker/wasm32-unknown-unknown/Dockerfile

@@ -9,6 +9,7 @@ RUN apt-get update -y && apt-get install -y --no-install-recommends \
   libc6-dev \
   make \
   python \
+  python3 \
   xz-utils
 
 # Install `wasm2wat`