Drop Python 3.6 Support

[gsheni]

• As numpy 1.20.0 drops support for python 3.6, we will be dropping support for it in an upcoming release.

[tyler3991]

ice boxing this issue on zenhub for now

[gsheni]

We can go ahead with this for now, as EvalML dropped Python 3.6 and Featuretools will be dropping support soon as well.

[tamargrey]

@gsheni I think we need to drop 3.6 support before we can add #264 because it'll require us to change our min pandas to 1.2.0, which only supports python 3.7 and up

[gsheni]

@tamargrey Gotcha. That's a good enough push to drop python 3.6 (especially since numpy, evalml have dropped support for it as well). Moved #530 into the top of our sprint.