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 98 downloads with pip from pypi out of
total of 15,357 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 Qiskit/qiskit#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]

It looks like we'll need to update the branch protection rules here to switch Mypy (3.5) to Mypy (3.6) but besides that this should be ready to go. Terra and Ignis have already merged their equivalent PRs Qiskit/qiskit#4926 and qiskit-community/qiskit-ignis#486