Fix pandas dependency

[95-martin-orion]

Error highlighted by @akushnarov: pandas requires NumPy >= 1.6.1 due to datetime64 dependency.

[95-martin-orion]

We use numpy.random.default_rng transitively via qiskit, and it was added in numpy 1.17, so that's our actual minimum version. The reason that numpy~=1.16 still passed our test suite is that ~= gets the latest compatible version (i.e. v1.*); however, it only guarantees that this version will be at least v1.16.

[95-martin-orion]

Okay, this is getting strange: numpy~=1.17 still gives the pandas requires NumPy >= 1.6.1 error.

I found this SO question which suggests that MacOS has a default numpy version that may be causing this. Recommended fix is pip install numpy --upgrade or manually deleting the default numpy install.

I can only assume that this didn't appear until now because of some change to pandas which increased its minimum numpy requirement.

[95-martin-orion]

Based on the explosion of package-version downloads in the logs and an apparent change from pip-20.2.4 to 20.3, I suspect this is actually caused by pypa/pip#9215. Closing in lieu of a more appropriate fix.

[95-martin-orion]

Closing in lieu of a more appropriate fix.

See #3574.

[mpharrigan]

Can we delete this branch?