Set version

Issue ronaldoussoren#281: App won't launch on macOS 10.9 or 10.10

Version 0.21

Don't advertise support for old python versions

Fix test failures

Remove explicit support for botocore, but autodetect vendored version…

…s of six

Set version number

Small metadata updates

Remove debug output

Improve detection of being launched by launchservices (for ArgvEmulator)