Add a frequenz top-level py.typed file

This can prevent some issues with editable installs and mypy, which
sometimes complains about a frequenz.xxx module not having the py.typed
file.

Usually this file is created automatically by pip when doing proper
installs, but when using editable installs that doesn't happen.

Signed-off-by: Leandro Lucarella <leandro.lucarella@frequenz.com>

Author: llucax