more code updates

- 2021 copyright
- remove unneeded compat functions
- remove unneeded filter constructs
- bytestring_passthrough mode doesn't actually do anything,
  as Py3k StringIO doesn't accept bytes in any case, remove this
- should we call ourselves 2.0 and do semver ?

Change-Id: I8a8cb5e78037b758e58c87d5d7b4f66b6c211419