Feature summary:
Find a new sse client package for Eventstreams. Possible options:

• sseclient-py (most common used)
• aiohttp-sse-client
• Support our own sse client which has the benefit that their usage does not depend on a virtual environment installation on toolforge (where sseclient isn't preinstalled)

Reason:
Currently we use sseclient packages for EventStreams but we had to pin its version to < 0.0.23 in rPWBC3babbc5 due to T222885. This packages is not activly maintained and the patch for T222885 (issue 28) is is pending for 3 years and other improvements aren't available. sseclient-py is the most common used client currently.