Resolves #2458

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