DEP: Python 3.6 support

[snowman2]

EOL is December 2021: https://devguide.python.org/#status-of-python-branches

• numpy dropped support June 2020 ref
• pandas dropped support June 2020 ref
• xarray dropped support in 0.17.0 (http://xarray.pydata.org/en/stable/whats-new.html#breaking-changes)
• conda-forge support will be dropped soon ref

I am thinking that 3.0.1 should be the last version to support python 3.6.