Change `setup.py` call w/ `pip install` @ tests

Using this CLI interface has been deprecated in `setuptools` [[1]].

[1]: https://blog.ganssle.io/articles/2021/10/setup-py-deprecated.html

testing/acceptance_test.py

@@ -1464,7 +1464,10 @@ 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