Remove Python 3.6 support

[wendycwong]

From discussion with @sebhrusen:

According to Seb, h2o-functions support need Python 3.7 or above. From our 1:1, we discuss the following:

1. Seb will open a branch to remove all python 3.6 stuff from h2o-3
2. Wendy will check with customers and make sure nobody is missing python 3.6.

When 2 is completed, Seb's work can be merged into H2O-3 repo.

[wendycwong]

Checked with Gen/Shivam, their customer uses R mainly. Don't care about Python.

[wendycwong]

Checking with Feng Bai, Karthik Kannappan, Arun, George, Bernard

[wendycwong]

Karthik Kannappan

Okay if it is only for newer H2O-3 version.
Everyone else I know is on 3.8+.
And generally okay with going 3.10+

[wendycwong]

George: His customer should be okay with Python 3.6 going away.

[wendycwong]

Feng Bai said his customers, WF and BA won't need 3.6 support in the future.