[webauthn] Upgrade Bikeshed to Python 3 (#1367)

emlun has just created a new issue for https://github.com/w3c/webauthn:

== Upgrade Bikeshed to Python 3 ==
Bikeshed is about to port to Python 3 soon, so it's likely we'll need to update our build scripts to match.

Please view or discuss this issue at https://github.com/w3c/webauthn/issues/1367 using your GitHub account