fix: prevent pytest from collecting tests on testing.TestClient #1344

Job	Run time
tox -e pep8 (3.10 on ubuntu-latest)	37s
tox -e blue (3.10 on ubuntu-latest)	37s
tox -e pep8-examples (3.10 on ubuntu-latest)	26s
tox -e pep8-docstrings (3.10 on ubuntu-latest)	33s
tox -e mypy (3.10 on ubuntu-latest)	48s
tox -e py310 (3.10 on ubuntu-latest)	2m 10s
tox -e py310_sans_msgpack (3.10 on ubuntu-latest)	54s
tox -e py310_cython (3.10 on ubuntu-latest)	3m 41s
tox -e docs (3.10 on ubuntu-latest)	1m 2s
tox -e towncrier (3.10 on ubuntu-latest)	57s
tox -e look (3.10 on ubuntu-latest)	34s
tox -e asgilook (3.10 on ubuntu-latest)	30s
tox -e check_vendored (3.10 on ubuntu-latest)	31s
tox -e twine_check (3.10 on ubuntu-latest)	31s
tox -e daphne (3.10 on ubuntu-latest)	2m 37s
tox -e hypercorn (3.10 on ubuntu-latest)	2m 28s
tox -e e2e_chrome (3.10 on ubuntu-latest)	1m 6s
tox -e e2e_firefox (3.10 on ubuntu-latest)	1m 24s
tox -e no_optional_packages (3.10 on ubuntu-latest)	1m 53s
tox -e pypy3 (pypy3.9 on ubuntu-latest)	3m 11s
tox -e py37 (3.7 on ubuntu-latest)	1m 49s
tox -e py38 (3.8 on ubuntu-latest)	2m 13s
tox -e py38_cython (3.8 on ubuntu-latest)	4m 10s
tox -e py39 (3.9 on ubuntu-latest)	1m 52s
tox -e py39_cython (3.9 on ubuntu-latest)	4m 50s
tox -e py311 (3.11 on ubuntu-latest)	1m 45s
tox -e py311_cython (3.11 on ubuntu-latest)	4m 51s
tox -e py310_nocover (3.10 on macos-latest)	2m 42s
tox -e py310_nocover (3.10 on windows-latest)	3m 54s
tox -e py38_smoke (3.8 on ubuntu-20.04)	1m 26s
tox -e py38_smoke_cython (3.8 on ubuntu-20.04)	3m 14s
tox -e wsgi_servers (3.8 on ubuntu-latest)	3m 8s
	1h 2m 24s

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