Pin PyOpenSSL<20 and prettytable<2 due to python2.7 conflicts

[cognifloyd]

In order to install st2 v3.3 requirements on python 2.7, we need to pin PyOpenSSL<20 and prettytable<2.

We require requests==2.23.0 which needs pyOpenSSL >= 0.14.
https://github.com/psf/requests/blob/v2.23.0/setup.py#L105

We also pin cryptography==2.8, but pyOpenSSL==20.0.0 was released Nov 27, 2020 and that requires cryptography>=3.2.
https://github.com/pyca/pyopenssl/blob/20.0.0/setup.py#L98

st2client requires prettytable. prettytable 2.0 was released in November as well, and it does not support python2. pip is still trying to install it though, so we need to prevent that.

[cognifloyd]

Fixing this is required to fix the exchange pack tests on python2.7.

For the pack tests, we don't need to cut a release. Later, we'll probably need to release v3.3.1 with this fix. Anyone that needs python2.7 can't upgrade to v3.4, and v3.3 won't install on python 2.7 without this fix.

[cognifloyd]

I’m trying to fix the packaging build. At first it was trying to use the master branch of st2-packaging to build this 3.3 only change. That checked for an rbac command that is only installed with the rbac package and that isn’t installed on 3.3, so I fixed that here: 97a0315

Now, el8 and bionic are passing on circleci, but both el7 and xenial are not.
They are both complaining that pip is not installed during “Build the ${DISTRO} Packages”. Any ideas?
https://app.circleci.com/pipelines/github/StackStorm/st2/948/workflows/07219ab8-90a7-403e-a1f8-adc553d3d6de/jobs/12289/parallel-runs/0?filterBy=ALL

Oh, and Travis was passing fine. One of the builds errored this time, but re-running should fix it.

[cognifloyd]

It turns out that packaging build images no longer have python2.7. So, since xenial and centos7 needed python2.7 to build, we can no longer build the packages.

Re-adding python2.7 would take significant effort and probably isn't worth it. See the discussion here:
StackStorm/community#40 (comment)