Build pystack DMWM package on python3

[amaltaro]

Impact of the new feature
WMCore services depending on pystack, which I believe to be almost all of them.

Is your feature request related to a problem? Please describe.
Yes, we need to be able to successfully install WMAgent on python3 stack.

Describe the solution you'd like
We have to either:

• create a new pystack spec which will depend on python3
• or investigate, and if possible, drop this pystack dependency once we move our services to python3
  such that we can successfully build and deploy WMAgent on a python3 stack.

This package is maintained by us, so if really needed, I think we could create a python3 branch and port the source code to python3.

Describe alternatives you've considered
More alternatives might appear as we start working on it.

Additional context
Issue spotted and mentioned here: #10302 (comment)

[amaltaro]

This cmsdist PR is meant to fix it: cms-sw/cmsdist#6714

with the complete removal of pystack dependency from our wmagentpy3 and wmagentpy3-dev spec files. It's still very hard to test how it will go because there are still other python2 dependencies breaking the agent deployment. So, just a heads up that we might have to come back to this issue...