Fix plone-stubs install: use git URL with Python >= 3.12 marker

plone-stubs is not published on PyPI, so reference it directly from
GitHub. Remove it from mx.ini (no marker support) and use a PEP 508
direct reference in pyproject.toml instead.

Author: ericof

mx.ini

@@ -11,7 +11,3 @@ version-overrides =
     pytest-plone>=1.0.0a1
     zope.pytestlayer>=8.3
     pytest>=8.4.0
-
-[plone-stubs]
-url = https://github.com/plone/plone-stubs.git
-pushurl = git@github.com:plone/plone-stubs.git

pyproject.toml

@@ -46,7 +46,7 @@ dependencies = [
 
 [project.optional-dependencies]
 test = [
-    "plone-stubs; python_version >= '3.12'",
+    "plone-stubs @ git+https://github.com/plone/plone-stubs.git ; python_version >= '3.12'",
     "pytest-cov",
     "pytest-xdist",
     "mypy>=1.15.0",
@@ -68,6 +68,9 @@ plone = "pytest_plone.fixtures"
 [tool.uv]
 managed = false
 
+[tool.hatch.metadata]
+allow-direct-references = true
+
 [tool.hatch.version]
 path = "src/pytest_plone/__init__.py"