Pylint ~= 2.1.1 is broken on isort v5

[DavidGarfinkle]

I'm opening this issue because https://github.com/shopify/panama python client depends on shopify_python and there's a dependency issue on pylint v2.1.1 that I think needs to be fixed here

Currently shopify_python requires pylint ~= 2.1.1. But, pylint v2.1.1 has this dependency issue on 'isort >= 4.2.5. There's no dependency constraint there, so eventually isort released v5. pylint 2.1.1 picks up the new version and breaks.

pylint fixed this by pinning their dependencies, like isort>=4.2.5,<5

More recently, pylint 2.6.0 supports isort v5 pylint-dev/pylint#3725

Let's upgrade pylint to the newest version and use a compatible release on major version rather than minor

i.e., pylint ~= 2.1 as opposed to pylint ~= 2.1.1