Fix PytestAssertRewriteWarning in tests when run in PyPy on Github Ac…

…tions.

See pytest-dev/pytest#1934.

Error message:
```
../../../../home/runner/work/pytest-snapshot/pytest-snapshot/.tox/pypy3-pytest6/lib/pypy3.10/site-packages/_pytest/config/__init__.py:1114
  /home/runner/work/pytest-snapshot/pytest-snapshot/.tox/pypy3-pytest6/lib/pypy3.10/site-packages/_pytest/config/__init__.py:1114: PytestAssertRewriteWarning: Module already imported so cannot be rewritten: pytest_snapshot
    self._mark_plugins_for_rewrite(hook)
```

tox.ini

@@ -21,7 +21,6 @@ deps =
 setenv =
     coverage: TEST_RUNNER=coverage run -m pytest
 
-usedevelop = True
 commands =
     coverage: coverage erase
     {env:TEST_RUNNER:pytest} {posargs:tests}