Migrate from setup.py to pyproject.toml (PEP 517 and PEP 621)

[Zeitsperre]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR addresses comments raised in Xclim : Xarray-based climate data analytics pyOpenSci/software-submission#73
• Tests for the changes have been added (for bug fixes / features)
  • (If applicable) Documentation has been added / updated (for bug fixes / features)
• HISTORY.rst has been updated (with summary of main changes)
  • Link to issue (:issue:number) and pull request (:pull:number) has been added

What kind of change does this PR introduce?

• setup.py is no longer used as the packaging system, pyproject.toml with flit is now the packaging engine.
• several configurations for linting tools (including Manifest.in) have either been rendered obsolete or been migrated into the pyproject.toml
• TOML checkers are now present in pre-commit

What other changes need to happen?

• package requirements need to be migrated into the package
• user-end installation tests need to be performed
• README and HISTORY require a pre-build step in order to properly be packaged and uploaded to PyPI. The ways this was done to date involved a hacky string replace step in setup.py. This might now need to be handled with a build-helper script
  • Drop the changelog from the PyPI index readme

Does this PR introduce a breaking change?

Yes. Very much. Installation of the package should be the same on the user end, but development tooling has changed significantly. Documentation will need to be updated accordingly.

Other information:

https://chadsmith.dev/python-packaging/

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[Zeitsperre]

This PR is ready for final review!

[aulemahal]

Looks good to me!
I have an unrelated suggestion and an alternative fix.

[Zeitsperre]

@aulemahal test-py39-opt-slow is consistently giving me the following:

FAILED xclim/testing/tests/test_sdba/test_properties.py::TestProperties::test_spatial_correlogram - AssertionError: 
Not equal to tolerance rtol=1e-07, atol=0

Mismatched elements: 5 / 5 (100%)
Max absolute difference: 7.40815654e-05
Max relative difference: 4.549936e-07
 x: array([ 26.543211,  67.716254, 108.889298, 150.062341, 191.235384])
 y: array([ 26.543199,  67.716227, 108.889254, 150.062282, 191.23531 ])

Is this just a matter of adjusting tolerances, or should I be investigating further?

[aulemahal]

argh.
Yes easiest would be to adjust the tolerances.