Fix apispec version conflict for tests.

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code docstring.
• Add an entry in CHANGES.md summarizing the change and linking to the issue.
• Add *Version changed* or *Version added* note in any relevant docs and docstring.
• Run pytest and tox, no tests failed.