Part 3/4 of #5380

This currently includes the commits of #5381 (part 1) and #5386 (part 2), because this code is needed for the CDP connection stuff. Once those PRs have been merged into master, I will rebase this branch.

So the only relevant commits of this PR are the ones after the merge commit. See the check marks of the HEAD commits of those other branches.

This

1. Adds the trio-websocket and typing-extensions runtime dependencies.
   • A bit unfortunate that we have to add another dependency for websockets, but it is what it is... Async I/O via trio is a requirement for a proper implementation.
   • typing-extensions is required, because I needed TypeAlias for making the CDPEventListener's typing work. This conveniently also adds Self and other things.
2. Implements the CDP connection and session management.
   • As mentioned in Implement data extraction via web browser (Chrome devtools protocol) #5380, the basic design is borrowed from the trio-chrome-devtools-protocol project, which uses the MIT license. This is properly annotated with the included license text. I applied several improvements and fixes to the code. This is all fully tested with full coverage.
   • Trio 0.22 currently requires the from __future__ import annotations workaround so the memory channels can have proper typing information, which is important. This will be resolved in their next release (they are waiting for the EOL of py37), but I don't think it's worth removing the import and bumping the version requirement once it's released.