Update WPTIncludeList

This is a preparation for updating wpt_tools to introduce the WPT's
WebTransport over HTTP/3 test server. Add files under
wpt/tools/webtransport. This CL doesn't update wpt_tools yet. A
subsequent CL will actually update wpt_tools.

This CL also removes pywebsocket3 from wpt_tools since we don't use them.
We use third_party/pywebsocket3 instead.

Bug: 1201569
Change-Id: Iaaf7f33b52d9cd2b2ae3d693b9d59491503fa218
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3141320


Reviewed-by: Weizhong Xia <weizhong@google.com>
Commit-Queue: Kenichi Ishibashi <bashi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#918498}