requirements-test: Pin pytest to < 7.0.0

Related to T3916

requirements-test.txt

-pytest
+pytest < 7.0.0  # v7.0.0 removed _pytest.tmpdir.TempdirFactory, which is used by some of the pytest plugins we use
 dulwich >= 0.18.7
 swh.loader.core
 swh.loader.git >= 0.8

[Phabricator Migration user]

Some references in the commit message have been migrated:

• T3916 is now swh/meta#3916 (closed)