Improve tooling for maintenance

[smithara]

These changes introduce two main things to move toward easier maintenance, inspired by https://github.com/scikit-hep/cookie:

• Add linting configured with .pre-commit-config.yml
  • This is optional for contributors, while I can simply apply it during a PR (or maybe automatically with https://pre-commit.ci/). There's several tools configured there but most important is black to enforce a standard code style.
• Modernise packaging to use flit and pyproject.toml
  • (also: ./viresclient/ is moved to ./src/viresclient/ and ./test/ to ./tests/)
  • A downside with flit is that it does not come with an automatic versioning system like setuptools_scm
• Switch dependencies to always accept higher versions (to be mitigated by running a weekly(?) test)

Some things to be added in the future:

• proper test suite to cover integration with the server, and regression checks
• type annotations and checking with mypy (?)

[pacesm]

Looks okay to me.