Drop support for Python 3.6

[mtreinish]

Summary

As has been advertised for some time the Qiskit 0.19.0 release is the
last release which supports Python 3.6 (which goes End of Life in
December). This commit drops support accordingly so that the minimum
supported Python version is 3.7.

Details and comments

[mtreinish]

People keep asking when this is going to happen so I decided to push a PR for it
so I can link people to it (and save some typing). But this is blocked from merging
until 0.19.0 is released (which should be soon). Once we bump the version number
to 0.20.0 on main we can merge this as the first commit for 0.20.0

[coveralls]

Pull Request Test Coverage Report for Build 1546759712

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 5 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.007%) to 83.094%

---

Files with Coverage Reduction	New Missed Lines	%
qiskit/extensions/quantum_initializer/uc.py	2	88.65%
qiskit/pulse/library/waveform.py	3	89.36%

Totals
Change from base Build 1546753217:	-0.007%
Covered Lines:	51667
Relevant Lines:	62179

---

💛 - Coveralls

[jakelishman]

(preemptive approval)

There are several goodies to do with type-checking that we'll be able to use once Python 3.7 support is dropped, including from __future__ import annotations, which removes all the need for string-ified type annotations.

From now on, we can also rely on dictionaries to be in insertion-order, which I think has come up before with respect to Result.get_counts() and things like that. We also get module-level __getattr__, which is something we've definitely had usecases for as well (though perhaps we should be cautious with that one, since it will have a tendency to break auto-complete).

[jakelishman]

Removing on hold, because this is now eligible for merge.