Release 1.3.0

[wnienhaus]

Now that S2/S3 support is finally done, a new release/version is due.

TODO:

• Contemplate, that because not everything is 100% backwards compatible, whether this should become version 2.0.0?
• Keep a record of all actual steps for later creating release documentation (see issue "how to release" docs #64)
• Update CHANGES files
• Test PyPi "test" release process
• Try PyPi "production" release with pre-release version number (to confirm it still works)
• Announce on various channels