🔥 Get rid of `setuptools` dev runtime dep

Said dependency was being listed as needed for development. However,
it was only called via the deprecated `setup.py` CLI interface in the
tests once. This patch replaces the invocation with `pip install` and
pulls it from the list of the development runtime dependencies.

pyproject.toml

@@ -60,7 +60,6 @@ optional-dependencies.dev = [
     "mock",
     "pygments>=2.7.2",
     "requests",
-    "setuptools",
     "xmlschema",
 ]
 urls.Changelog = "https://docs.pytest.org/en/stable/changelog.html"

testing/acceptance_test.py

@@ -1464,7 +1464,7 @@ def test_issue_9765(pytester: Pytester) -> None:
         }
     )
 
-    subprocess.run([sys.executable, "setup.py", "develop"], check=True)
+    subprocess.run([sys.executable, "-Im", "pip", "install", "-e", "."], check=True)
     try:
         # We are using subprocess.run rather than pytester.run on purpose.
         # pytester.run is adding the current directory to PYTHONPATH which avoids