Avoid setting the multiprocessing context when importing lightning (#18177)

* avoid import at top module

* tests

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* remove comment

* update docs

* changelog

* mypy

* trigger app tests

* can't import lightning on py 3.8

* Update .github/workflows/ci-tests-app.yml

Co-authored-by: Jirka Borovec <6035284+Borda@users.noreply.github.com>

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Jirka Borovec <6035284+Borda@users.noreply.github.com>