Drop support for python 3.5

[mtreinish]

Summary

This commit drops support for running with python 3.5. It marks the
minimum supported version of the package as python 3.6, removes python
3.5 package pins, removes the 3.5 CI jobs, and removes the warning on
python 3.5. Looking at the PyPI stats since the deprecation period
started the number of users on Python 3.5 has diminished significantly,
but not disappeared. There were 783 downloads with pip from pypi out of
total of 25782 total pip downloads in the last 30 days. Compared to the
roughly 10% figure when we deprecated Python 3.5.

Merging this means we can not release until after the documented EoL
date for Python 3.5 support of September 13. This shouldn't be a problem
because with #4767 we will need to coordinate the release of all the
qiskit elements and are planning to do that after 09/13/2020.

It's worth noting that we should start planning to deprecate python 3.6
support sooner rather than later it goes EoL upstream at the end of
next year [1] and some of our other upstream dependencies (mainly numpy
et al) are going to remove support before the upstream Python EoL date
[2].

Details and comments

[1] https://devguide.python.org/#branchstatus
[2] https://numpy.org/neps/nep-0029-deprecation_policy.html

[mtreinish]

I'm leaving this on hold until we've discussed the 0.16.0 release plan and coordination with the other qiskit elements around #4767 . But, assuming my plan is tenable we will remove the namespace packaging and coordinate a release with all the elements from their master branch in late September then we can merge this on master safetly without breaking our advertised python 3.5 drop date since it won't be released until after the 3.5 EoL. Otherwise this will have to wait until after 9/13/2020. It's worth noting that even if this merges any backports we make to stable/0.15 will still have to support 3.5 which means we might have to manual change things as we backport fixes.