Consider pyproject.toml files for config if no other config files were found (#11962)

Today `pyproject.toml` is the standard for declaring a Python project root, so seems reasonable to consider it for the ini configuration (and specially `rootdir`) in case we do not find other suitable candidates.

Related to #11311

doc/en/reference/customize.rst

@@ -177,13 +177,20 @@ Files will only be matched for configuration if:
 * ``tox.ini``: contains a ``[pytest]`` section.
 * ``setup.cfg``: contains a ``[tool:pytest]`` section.
 
+Finally, a ``pyproject.toml`` file will be considered the ``configfile`` if no other match was found, in this case
+even if it does not contain a ``[tool.pytest.ini_options]`` table (this was added in ``8.1``).
+
 The files are considered in the order above. Options from multiple ``configfiles`` candidates
 are never merged - the first match wins.
 
+The configuration file also determines the value of the ``rootpath``.
+
 The :class:`Config <pytest.Config>` object (accessible via hooks or through the :fixture:`pytestconfig` fixture)
 will subsequently carry these attributes:
 
-- :attr:`config.rootpath <pytest.Config.rootpath>`: the determined root directory, guaranteed to exist.
+- :attr:`config.rootpath <pytest.Config.rootpath>`: the determined root directory, guaranteed to exist. It is used as
+  a reference directory for constructing test addresses ("nodeids") and can be used also by plugins for storing
+  per-testrun information.
 
 - :attr:`config.inipath <pytest.Config.inipath>`: the determined ``configfile``, may be ``None``
   (it is named ``inipath`` for historical reasons).
@@ -193,9 +200,7 @@ will subsequently carry these attributes:
     versions of the older ``config.rootdir`` and ``config.inifile``, which have type
     ``py.path.local``, and still exist for backward compatibility.
 
-The ``rootdir`` is used as a reference directory for constructing test
-addresses ("nodeids") and can be used also by plugins for storing
-per-testrun information.
+
 
 Example: