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* Use `extend-ignore` in flake8 config

This option allows to add extra ignored rules to the default list
instead of replacing it.

The default exclusions are: E121, E123, E126, E226, E24, E704,
W503 and W504.

Fixes #28.

Refs:
* https://github.com/pypa/setuptools/pull/2486/files#r541943356
* https://flake8.pycqa.org/en/latest/user/options.html#cmdoption-flake8-extend-ignore
*
https://flake8.pycqa.org/en/latest/user/options.html#cmdoption-flake8-ignore



* Enable complexity limit. Fixes jaraco/skeleton#34.

* Replace pep517.build with build (#37)

* Replace pep517.build with build

Resolves #30

* Prefer simple usage
Co-authored-by: Jason R. Coombs <jaraco@jaraco.com>

* Use license_files instead of license_file in meta (#35)

Singular `license_file` is deprecated since wheel v0.32.0.

Refs:
* https://wheel.readthedocs.io/en/stable/news.html
* https://wheel.readthedocs.io/en/stable/user_guide.html#including-license-files-in-the-generated-wheel-file

Co-authored-by: Jason R. Coombs <jaraco@jaraco.com>