Use extend-ignore in flake8 config (pettazz#33)

* 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 pettazz#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 (pettazz#37)

* Replace pep517.build with build

Resolves pettazz#30

* Prefer simple usage

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

* Use license_files instead of license_file in meta (pettazz#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>

.flake8

@@ -4,10 +4,6 @@ max-line-length = 88
 # jaraco/skeleton#34
 max-complexity = 10
 
-ignore =
-	# W503 violates spec https://github.com/PyCQA/pycodestyle/issues/513
-	W503
-	# W504 has issues https://github.com/OCA/maintainer-quality-tools/issues/545
-	W504
+extend-ignore =
 	# Black creates whitespace before colon
 	E203