DevOps: adopt PEP 621 and move build spec to `pyproject.toml`

[sphuber]

Following PEP 621 it is now possible to fully define the build procedure of your package in pyproject.toml. Since this PEP is now well supported, and for example pip can use it, we migrate to it since it provides a bunch of benefits:

• No longer need the deprecated setup.py to install the package, nor the custom setup.json.
• Version number and description are now dynamically fetched from the package. So there is no need to manually update the setup.json and there is no need for the custom version validation utility script.
• The MANIFEST.in is no longer necessary. The flit build tool will automatically include anything. The pyproject.toml has a tool section for flit that excludes the tests and docs folders.
  • All configuration of tools are consolidated in pyproject.toml. This allows to remove .style.yapf and configuration options in the .pre-commit-config.yaml.

Also the LICENSE file is renamed to LICENSE.txt to use a correct file extension.

[sphuber]

The changes in this PR will allow to add an automatic continuous deployment workflow. This will automate the release procedure. Simply push a new tag with a new version number and the workflow will automatically run the pre-commit and tests, and when they pass build the package and push to PyPI.