Update pytest-dev requirements

- be more vague about "packaging metadata" over explicitly mentioning `setup.py`
  (such that `pyproject.toml`-based distributions are allowed)
- drop extensions on `README.txt` / `LICENSE.txt` (it's more common to have `.md`
  / `.rst` / no extension)
- remove duplicate mention of license packaging metadata

CONTRIBUTING.rst

@@ -148,16 +148,16 @@ You can submit your plugin by subscribing to the `pytest-dev mail list
 mail pointing to your existing pytest plugin repository which must have
 the following:
 
-- PyPI presence with a ``setup.py`` that contains a license, ``pytest-``
+- PyPI presence with packaging metadata that contains a ``pytest-``
   prefixed name, version number, authors, short and long description.
 
 - a ``tox.ini`` for running tests using `tox <https://tox.readthedocs.io>`_.
 
-- a ``README.txt`` describing how to use the plugin and on which
+- a ``README`` describing how to use the plugin and on which
   platforms it runs.
 
-- a ``LICENSE.txt`` file or equivalent containing the licensing
-  information, with matching info in ``setup.py``.
+- a ``LICENSE`` file containing the licensing information, with
+  matching info in its packaging metadata.
 
 - an issue tracker for bug reports and enhancement requests.