Bump txaio from 18.8.1 to 20.4.1

Bumps [txaio](https://github.com/crossbario/txaio) from 18.8.1 to 20.4.1.
- [Release notes](https://github.com/crossbario/txaio/releases)
- [Changelog](https://github.com/crossbario/txaio/blob/master/docs/releases.rst)
- [Commits](crossbario/txaio@v18.8.1...v20.4.1)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

requirements/base.txt

@@ -47,7 +47,7 @@ redis==3.3.8  # https://github.com/antirez/redis
 sentry-asgi==0.2.0
 sentry-sdk==0.12.2  # https://github.com/getsentry/sentry-python
 twisted==20.3.0
-txaio==18.8.1
+txaio==20.4.1
 typing-extensions==3.7.4
 websocket-client==0.56.0 # need this for now to profile and find speedups
 websockets==8.0.2