Add more static code analysis

[Santobert]

This PR adds codespell, isort, flake8 and bandit:

• codespell finds spelling errors
• isort sorts imports based on their category 'STDLIB', 'THIRDPARTY', 'FIRSTPARTY', ...
• flake8 is a linter that covers some things not handled by pylint
• bandit looks for security issues

@stianaske please be honest. If you don't want your contributors to be bothered by these tools, I will remove them again. But I personally like the support provided by these utilities. I run them in my development environment anyway and would be happy to have a CI automate that for me

[stianaske]

In general I'm positive to using tools to ensure consistency across the codebase.

I do not have any experience with these tools, but let's try them out, and if the barrier to contributing becomes excessive we just remove them again.