Parallelize the test runs via pytest-xdist

Extract workaround for pytest-dev/pytest-xdist#376 as a fixture. Invoke the repair at the session level and only when xdist is present.

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