Change browser-demo build.sh to use python 3 again (#6992)

substrate/bin/node/cli/browser-demo/build.sh

 #!/usr/bin/env sh
 cargo +nightly build --release -p node-cli --target wasm32-unknown-unknown --no-default-features --features browser -Z features=itarget
 wasm-bindgen ../../../../target/wasm32-unknown-unknown/release/node_cli.wasm --out-dir pkg --target web
-python -m SimpleHTTPServer 8000
+python -m http.server 8000