Drop Python 2.7 support

[adamchainz]

Upstream IPython dropped it in version 6.0.0, in April 2017. I think it's fine to drop it here now.

Would also be good to use this opportunity to mark the release as 1.0.0.

[gotcha]

I would like to keep 2.7 support unless it avoids support for new features...

[adamchainz]

Why? It's dead for nearly 2 years now. For those on Python 2, pip will install the old version, thanks to python_requires.

[gotcha]

As said above, if it does not hinder new features, I see no reason to stop support.

Further, I know a lot of people still using Py 2.

[adamchainz]

Okay well up to you. You can keep this PR open for when the time comes if you like.

[gotcha]

Good point

[malkstar]

2.7 IPython vulernability fixes won't be installable through pip for https://github.com/gotcha/ipdb/pull/248, as they're installable from source only.

[ssbarnea]

@gotcha merge it please! Let's avoid disturbing the dead, or better avoid being hunted by zombies.