Python 3.6 End of Life

[amcintosh]

PSA that python 3.6 goes EOL as of December 23, 2021.

We will be dropping support sometime around that date. The 1.0.0 release will likely be breaking for python 3.6 as we plan to remove backports-datetime-fromisoformat from the project.

[amcintosh]

Dropping support for python 3.6 here: #28