Drop setuptools for flit and pyproject.toml

[Zeitsperre]

Changes

• Drop setup.py and move configurations into pyproject.toml using the flit backend
• Update testing to remove DeprecationWarnings and support newer Python version testing
• Update README.rst to reflect changes in this cookiecutter
• Add new hooks and remove obsolete hooks in pre-commit configuration
• Unpin most build dependencies
• Remove Travis CI configurations
• Removed all mentions of setup.py in tooling
• Updated cookiecutter parent-level documentation
• Added a new option uses_conda to determine whether conda-related documentation should be added
• Added workflows for bumping version, staging and production controls, and automatically updating actions versions

[Zeitsperre]

@RondeauG

No need to go too in-depth, but you mentioned that there were some documentation changes that you wanted to make. Feel free to do so here, or simply give me an idea, and I'll see what I can do.

[RondeauG]

Looks good!