Modernize the skipped `test_issue_9765` regression test (#12468)

* 🚑 Explicitly set encoding @ `subprocess.run()`

This invocation is present in the `test_issue_9765` regression test
that is always skipped unconditionally, resulting in the
`EncodingWarning` never manifesting itself in CI or development
environments of the contributors.

* 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

@@ -1464,14 +1464,21 @@ 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
         # the bug. We also use pytest rather than python -m pytest for the same
         # PYTHONPATH reason.
         subprocess.run(
-            ["pytest", "my_package"], capture_output=True, check=True, text=True
+            ["pytest", "my_package"],
+            capture_output=True,
+            check=True,
+            encoding="utf-8",
+            text=True,
         )
     except subprocess.CalledProcessError as exc:
         raise AssertionError(