Repair Descriptions with newlines and emit a warning

Repair Descriptions with newlines and emit a warning that the value will be disallowed in the future.

This reverts commit ef457b2e, reversing
changes made to d2b1f7eb.

Description cannot contain a newline.

Fix .egg-info metadata support for zipimport

Co-authored-by: Sviatoslav Sydorenko <wk.cvs.github@sydorenko.org.ua>

Clarify Development Mode first paragraph

docs (build_meta): fix spelling mistake

Migrate to `extend-exclude` in flake8 config

Fix misplaced/mistyped per-file-ignores in flake8

Fix code typo in entry_point.rst

Use `extend-ignore` in flake8 config

Use license_files instead of license_file in meta

Define create_module()/exec_module() in VendorImporter

Enable McCabe complexity check in flake8

Fix/2529

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

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

* Replace pep517.build with build

Resolves #30

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