Fix pytest warning when EARL reporting plugin in loaded
Fix pytest warning when EARL reporting plugin in loaded

This eliminates this warning:
```
.venv/lib64/python3.7/site-packages/_pytest/config/__init__.py:676
/home/iwana/sw/d/github.com/iafork/rdflib/.venv/lib64/python3.7/site-packages/_pytest/config/__init__.py:676: PytestAssertRewriteWarning: Module already imported so cannot be rewritten: test.earl
  self.import_plugin(import_spec)
```

By adding PYTEST_DONT_REWRITE to the plugin module docstring.

RDFLib/rdflib,RDFLib/rdflib,RDFLib/rdflib,RDFLib/rdflib