Remove Python 3.6 support

[tholor]

Proposed changes:

• Let's drop the support for python 3.6 as it causes issues with some dependencies (e.g. faiss) and we want to be able to use py37 features like dataclasses without any headache about backwards compatibility.

Status (please check what you already did):

• First draft (up for discussions & feedback)
• Final code

Fixes #1056

[lalitpagaria]

Just suggestion to add support to perform tests on 3.7+ along with multiple OSs. It is very easy to implement.
Windows CI can be made experimental if you don't want to actively support it.

Sharing Obsei's job for reference
CI: https://github.com/lalitpagaria/obsei/blob/master/.github/workflows/build.yml
Support Matrix: https://github.com/lalitpagaria/obsei#support-and-release-matrix

[lalitpagaria]

Should we add breaking change label?
I am not sure as this will not break anything for 3.7+ but only remove support for 3.6