Support static type checking with `mypy`

[mlincett]

This PR provides a minimal set of change to let a mypy run pass on the current codebase. The primary reason for this is to allow the use of mypy to test improvements and refactors of the existing code.

Note that mypy (in my understanding) checks for type consistency only functions that have a typed return value, and optionally all functions with at least one type annotation even if untyped. It also does more general QC, function signature-based syntax checking. Notably it does not like variable name re-definitions.

This PR also deprecates python 3.8 and closes #301 so we can use native tuple and list type annotations.

To summarise the changes, they mostly fall in one of these categories:

• type annotations for the subclasses dictionary we use in several flarestack.core functionalities;
• style improvements while I was at it (f-strings, alphabetical order of imports etc.)
• type annotation with Optional[T] for variables that can be of type T or None; on the long term, conditional statements based on None values should be likely deprecated;
• commenting out of code affected by non-obvious syntax errors: there are several code blocks under misc or utils that are no longer compatible with the current flarestack core API and that are evidently untested (otherwise their test would fail);
• avoid variable redefinitions such as x = np.array(x) since a variable should not change its type; this may seem a bit too strict but on the long term it improves clarity.

[robertdstein]

I updated the CI. It's under settings, then Branch protection rules. I removed python 3.8, and added python 3.11, as a required check.

[mlincett]

I updated the CI. It's under settings, then Branch protection rules. I removed python 3.8, and added python 3.11, as a required check.

Looked for it but could not find it. Apparently I don't have high enough privileges (I guess).