Update test.yml to remove python 3.8

[justinpolygon]

With the latest websockets updates in #935 python 3.8 is not longer supported. We need to move that to 3.9 but that's still 5 versions back.

[penelopus]

is this in response to the failing test in #935?

• seeing a pending test that hasn't finished and seems to be pending (but also expected if 3.8 was removed?)