Drop Python 3.6 support

[Saransh-cpp]

Just saw scikit-hep/awkward#1661

I remember @henryiii mentioning dropping Python 3.6 support. Maybe this should be dropped in Vector v1.0?

[jpivarski]

I know he's in favor of dropping it everywhere and getting into a regular schedule of adopting/dropping Python versions according to coordinated standards (e.g. NEP 29 and Scikit-HEP developer pages).

It's best, however, to drop a Python version while not also introducing a lot of other changes, so that users don't have to choose between features that are known to work in Python 3.6 (because your tests against Vector's main pass) and being able to use Python 3.6.

They should probably also align with at least minor version updates. You have a lot of changes since 0.8.5, so you'd want to release 0.9.0 with Python 3.6 support, so that users can get these updates even if they can't give up Python 3.6, and then immediately release 0.10.0 without Python 3.6 support and develop from there, so that constraints on what versions people can use are as independent of each other as possible. Version 1.0.0 will include other big changes.

[Saransh-cpp]

Oh, that makes sense, thanks! I will remove the support once v0.9.0 is out, and tag a new version along with the removal (v0.10.0).