Support external tox.pytest usage via "test" extra (#3422)

In order to allow plugin writers to easily pull in all dependencies
needed to use the `tox.pytest` module, add a new `test` extra with a
subset of the test dependency group. Though the pytest package should
be present in the users' environment anyway, we still add it for
consistency and to produce a conflict just in case the version has
been constrained to an older version by the project.

docs/changelog/3415.misc.rst

@@ -0,0 +1 @@
+Provide ``test`` extra to support plugin authors using ``tox.pytest``.

pyproject.toml

@@ -62,6 +62,11 @@ dependencies = [
   "typing-extensions>=4.12.2; python_version<'3.11'",
   "virtualenv>=20.26.6",
 ]
+optional-dependencies.test = [
+  "devpi-process>=1.0.2",
+  "pytest>=8.3.3",
+  "pytest-mock>=3.14",
+]
 urls.Documentation = "https://tox.wiki"
 urls.Homepage = "http://tox.readthedocs.org"
 urls."Release Notes" = "https://tox.wiki/en/latest/changelog.html"