build: implement pep621

[kuwv]

Switch invoke to pep621.

Moved invoke => src for various improvements with setuptools and tox.

Need to update orb to work with pep621. This should work with circleci but untested:

---
parameters:
  config:
    ...
    default: pyproject.toml
steps:
  ...
  - run:
      ...
      command: |
        pip install .[dev]
        ...

[kuwv]

@bitprophet I originally created this PR: https://github.com/pyinvoke/invoke/pull/767. But I think this is probably less disruptive and more inline with the current PyPI packaging recommendations.

[LecrisUT]

Any links to why the standard src/module_name structure? I didn't see it in PEP621

[kuwv]

Here you go: https://packaging.python.org/en/latest/discussions/src-layout-vs-flat-layout/

[LecrisUT]

Thanks, this makes sense

[LecrisUT]

Check the Circle-CI. You need to link the cache key to pyproject.toml instead of dev-requirements.txt.

Seems like it needs to be re-written with something like this