Configure automatic code linter

[secure-systems-lab]

Configure automatic code linter

[MVrachev]

Do we want to mention mypy as well and start using type annotations?
It's possible now when securesystemslib doesn't use python2.

[lukpueh]

python-tuf uses black, isort and pylint these days and I'm very happy with the setup because it has eliminated code style discussions (see invocation and config).

Let's do the same thing at least for any newly added modules in securesystemslib.

[trishankatdatadog]

I propose trying GitHub's LGTM static analysis tool: https://lgtm.com/projects/g/theupdateframework/tuf/?mode=list

[joshuagl]

💯

[jku]

I propose trying GitHub's LGTM static analysis tool: https://lgtm.com/projects/g/theupdateframework/tuf/?mode=list

this is irrelevant to the discussion but: the errors on the linked page are incorrect as all classes are "new-style" in Python now, explicitly deriving from object is not required (the distinction was a python 2 workaround).