fix: prevent pytest from collecting tests on testing.TestClient #1343

Job	Run time
tox -e pep8 (3.10 on ubuntu-latest)	39s
tox -e blue (3.10 on ubuntu-latest)	36s
tox -e pep8-examples (3.10 on ubuntu-latest)	29s
tox -e pep8-docstrings (3.10 on ubuntu-latest)	30s
tox -e mypy (3.10 on ubuntu-latest)	54s
tox -e py310 (3.10 on ubuntu-latest)	2m 42s
tox -e py310_sans_msgpack (3.10 on ubuntu-latest)	38s
tox -e py310_cython (3.10 on ubuntu-latest)	4m 31s
tox -e docs (3.10 on ubuntu-latest)	44s
tox -e towncrier (3.10 on ubuntu-latest)	46s
tox -e look (3.10 on ubuntu-latest)	28s
tox -e asgilook (3.10 on ubuntu-latest)	30s
tox -e check_vendored (3.10 on ubuntu-latest)	27s
tox -e twine_check (3.10 on ubuntu-latest)	41s
tox -e daphne (3.10 on ubuntu-latest)	3m 9s
tox -e hypercorn (3.10 on ubuntu-latest)	2m 33s
tox -e e2e_chrome (3.10 on ubuntu-latest)	1m 10s
tox -e e2e_firefox (3.10 on ubuntu-latest)	1m 8s
tox -e no_optional_packages (3.10 on ubuntu-latest)	1m 50s
tox -e pypy3 (pypy3.9 on ubuntu-latest)	3m 5s
tox -e py37 (3.7 on ubuntu-latest)	1m 48s
tox -e py38 (3.8 on ubuntu-latest)	2m 17s
tox -e py38_cython (3.8 on ubuntu-latest)	3m 47s
tox -e py39 (3.9 on ubuntu-latest)	2m 4s
tox -e py39_cython (3.9 on ubuntu-latest)	3m 44s
tox -e py311 (3.11 on ubuntu-latest)	2m 11s
tox -e py311_cython (3.11 on ubuntu-latest)	4m 43s
tox -e py310_nocover (3.10 on macos-latest)	2m 59s
tox -e py310_nocover (3.10 on windows-latest)	2m 57s
tox -e py38_smoke (3.8 on ubuntu-20.04)	1m 5s
tox -e py38_smoke_cython (3.8 on ubuntu-20.04)	3m 15s
tox -e wsgi_servers (3.8 on ubuntu-latest)	3m 45s
	1h 2m 5s

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