Drop Python 3.6 support

[dolfinus]

🚀 Feature Request

How about dropping Python 3.6 support?

Motivation

Is your feature request related to a problem? Please describe.

Python 3.6 reached end of life 23 Dec 2021, and will no longer receive updates and bugfixes

Pitch

Describe the solution you'd like

Let's drop Python 3.6 support from the package and from CI/linters/etc.

[Jasha10]

@dolfinus see omry/omegaconf#791.

[pixelb]

Note we can monitor python minor version usage at:
https://pypistats.org/packages/hydra-core

[pixelb]

Also it's worth noting that python 3.6 is the centos 8 official version, which means longer support than would normally be provided a version

[dolfinus]

Also it's worth noting that python 3.6 is the centos 8 official version

CentOS 8 have a python3.8 package as well as 3.6. Also in case of development environment, pyenv is a common solution which allows to use any python version user wants without making any changes to list of OS packages.

[Jasha10]

It's become onerous to support some CI with python3.6. Mypy no longer has official support for 3.6.
In PR #2472 I disable lints for python3.6. The core tests are still working well enough with 3.6, so I'll leave those intact for now.