Python 3 style cleanup (part 2)

[pajod]

Cleanup, follow-up to #3189 - There is a change of config default in here, but for our workers that only impacts who sees the warning about the ignored setting. Rest is intended to be no-op.

• Unfinished consideration: InvalidChunkSize (and most of the meaning of NoMoreData and ChunkMissingTerminator, though that needs to be split first) subclassing IOError is just wrong, replacing with OSError does not fix that. Input reading worked perfectly fine, its the content of that input that our parser found issues with. Maybe some of it could re-use the built-in EOFError, but only while avoiding confusing unexpected SSL tunnel shutdown with previously announced end of input.

• Suggested merge order: merge last