Make st2client setup.py requirements consistent

[LindsayHill]

Installing st2client using pip install st2client has missing dependencies. It is unusable until you run pip install argcomplete pytz jsonschema prompt_toolkit python-editor.

This PR makes the setup.py requirements consistent with those in requirements.txt.

Fixes #3367

[bigmstone]

@Kami Is the reason you disabled pulling from requirements.txt in #2423 still valid or can we re-enable that? It'd be a better fix than trying to manually keep st2client in sync.

[LindsayHill]

FWIW, I would be much happier about having this auto-synced. I'm just trying to deal with some immediate pain, but a 'proper' fix would be welcome.

[Kami]

We fixed dist_utils.py so it also works with pip 10.0.0 so I would be OK with reading from requirements.txt again if we can indeed verify it works correctly.

Still need to verify it works with various versions of pip (pre 9, 9, and 10). I believe in the past there were issues with older versions of pip.

[Kami]

I did some digging in and testing - we now install pip 9.0.1 inside st2 and pack virtual environments so I believe we should be fine.

It doesn't work with very ancient versions of pip, but I just checked and most other software also doesn't support pip < 9.0 so we should be fine.

I will push a change which updates it to use requirements.txt again.

@LindsayHill thanks for opening this PR and having us reevaluate this decision again.

I also tagged this issue with 2.9.0 and will wait with merging this we have a branch for v2.8.

[Kami]

I pushed various changes which should resolve that - f9455e2, 8c6659c, 79d0d13.

It should work with pip >= 6.0.0, but we really shouldn't support anything < 9.0.0 so if a user complains, we should ask them to upgrade / use more recent version of pip.

[LindsayHill]

Thanks for that @Kami. Totally fine for this to be 2.9.0. It's not a 2.8.0 blocker.

[Kami]

I pushed some more changes / fixes to this branch and confirmed it indeed fixes issue reported in #3367 - #3367 (comment).