Refactoring to pass linting rules

[s3alfisc]

@NKeleher has suggested the following linting rules:

select = [
  "F", # Pyflakes
  "E", # pycodestyle errors
  "W", # pycodestyle warnings
  "I", # isort
  "D", # flake8-docstrings
  "UP", # pyupgrade
  "SIM", # flake8-simplify
  "TRY", # tryceratops
]

and suggests to break up the refactoring into module specific PRs which we merge into fix-linting-errors step-by-step.

• [ ] utils
• [ ] demean
• [ ] model_matrix_fixest
• [ ] feols
• [ ] fepois
• [ ] did/*
• [ ] summarize
• [ ] visualize
• [ ] FixestMulti - mostly docstyle and upgrade
• [ ] FormulaParser - mostly docstyle and upgrade
• [ ] estimation - mostly docstyle and exception handling

Then when all of these are linted, we merge fix-linting-errors to the master branch.

[NKeleher]

Perfect! Thanks @s3alfisc. I've started working these items.

[s3alfisc]

As I have merged all changes to master with 5408f25, this can be closed! Thanks for all your work on this @NKeleher 🎉