Improve type hints nullable parameters

[druizz90]

Improve type hints for arguments that are nullables and fix flake8 pre-commit hook.

Changes proposed in this pull request:

• Update pre-commit configuration with the latest version of flake8 to prevent errors
• Improve type hints for parameters that are nullables

[coveralls]

coverage: 94.15% (+0.003%) from 94.147%
when pulling b2de340 on druizz90:improve-type-hints-nullable-strings
into be9b280 on spec-first:main.

[RobbeSneyders]

Thanks @druizz90!