Maintenance cleanup

[blink1073]

• Remove flake8 file
• Fix version handling in docs
• Add readthedocs config
• Build docs with warnings check in CI
• Add pydata-sphinx-theme
• Add hatch envs for test and docs build

[blink1073]

@takluyver can you please add blink1073 as a maintainer on https://readthedocs.org/projects/terminado/?