Eventsource tests are broken under python3

[jdm]

Judging by the comment in #23284 this is expected. It broke Servo's nightly sync, however, since we're running a small set of tests (/eventsource) under python3 to verify the rest of our test infrastructure still works.

[stephenmcgruer]

cc @Hexcles @jgraham as an FYI

This will eventually be fixed as we upgrade handlers to Python 3, but perhaps we could fix the eventsource/ ones ahead of web-platform-tests/rfcs#49 being ratified given that we're sort of regressing here?

@jdm - does it need to be those tests you run, or could you run some that don't depend on the python file handler support?

[jdm]

Running any small set of tests is fine for our purposes. I nearly have all of the handlers fixed to get the tests passing again, but there are a couple stragglers that do special things with encodings, of course.

[Hexcles]

@jdm has #24184 fixed this issue?

[jdm]

Yep.