DEPS: drop 3.6

[jreback]

according to schedule we should 3.6 in june 2020

https://numpy.org/neps/nep-0029-deprecation_policy.html

prob ok to keep for the 1.1 release then should drop for 1.2. we will likely be adding. 3.9 support so this makes sense.

[alimcmaster1]

Happy to help here when the time comes :)

[simonjayhawkins]

prob ok to keep for the 1.1 release then should drop for 1.2. we will likely be adding. 3.9 support so this makes sense.

we had a discussion about Python version support in #28415, so if doing this we need to update the version policy https://pandas.pydata.org/docs/development/policies.html#python-support