remove six: Replace six.string_types and six.integer_types, etc.

[dbieber]

This PR includes:

• assuming Python 3 for test skipping
• assuming Python 3 for fire's various type checks
• assuming Python 3 for imports (like asyncio)
• assuming Python 3 for getting function signatures
• six is no longer considered a hidden module (and so if a user of fire has six in their globals when they call Fire(), six will now show up where it used to be hidden)

This PR does not remove six from console/ code.