Minor fixes in documentation and release:prepare #596
Description
Summary
In #590, we modified the pyupgrade feature, but we missed getting the documentation. This should be fixed to reflect the current status: https://exasol.github.io/python-toolbox/main/user_guide/features/formatting_code/index.html
It's been noted throughout several reviews the versioned changelog lacks a newline between the topmost heading and the first line of content. This and its test should be adapted to reflect this.