chore(deps): update websockets requirement from <15 to <16

Updates the requirements on [websockets](https://github.com/python-websockets/websockets) to permit the latest version.
- [Release notes](https://github.com/python-websockets/websockets/releases)
- [Commits](python-websockets/websockets@1.0...15.0)

---
updated-dependencies:
- dependency-name: websockets
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

requirements/async.txt

@@ -1,3 +1,3 @@
 # pip install -r requirements/async.txt
 aiohttp>=3,<4
-websockets<15
+websockets<16