Drop support for Python 2

[mjs]

Python 2 is no longer supported upstream. I propose that the next major release of IMAPClient (3.0) no longer supports Python 3. This could coincide with the imaplib removal changes in #400.

If we go ahead with this, the following changes should be made:

• Release all unreleased changes as the final Python 2 supporting version
• Update the docs and README to mention the last version of IMAPClient that does support Python 2
• Update setup.py
• Update .travis.yml
• Remove the use of six and hacks to support a mixed 2/3 codebase (this can be done gradually)

@NicolasLM what do you think?

[NicolasLM]

@mjs It makes sense to drop support for Python 2 now.

[NicolasLM]

Hey @mjs, @JohnVillalovos' PR removing Python 2 looks good. Do you want to publish a final release supporting Python 2.7 before we merge it?

[mjs]

I was planning on making 2.3.0 that last Python 2 supporting release but I haven't put much time into it and there's nothing there that needs to be in a Python 2 supporting release.

I don't want to hold up the Python 2 removal work so let's get #474 merged. I will review now.