[infra] Port serve.py from Python2 to 3

These scripts are useful when testing WebAssembly locally
because the mimetype impacts how the binaries are loaded.

The porting was achieved by doing the following:
python -m lib2to3 -w -n serve.py


Change-Id: I09673fa881339a9b157c5fc993e190766efcd85e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/443884
Reviewed-by: Erik Rose <erikrose@google.com>
12 files changed