pytester: avoid EncodingWarning from `locale.getpreferredencoding`

When running `tox -e py-lsof` I get a deluge of this warning:

```
src/pytest/.tox/py-lsof-numpy-pexpect/lib/python3.11/site-packages/_pytest/pytester.py:130: EncodingWarning: UTF-8 Mode affects locale.getpreferredencoding(). Consider locale.getencoding() instead.
```

Use `locale.getencoding` instead.

src/_pytest/pytester.py

@@ -121,13 +121,18 @@ def pytest_configure(config: Config) -> None:
 
 class LsofFdLeakChecker:
     def get_open_files(self) -> List[Tuple[str, str]]:
+        if sys.version_info >= (3, 11):
+            # New in Python 3.11, ignores utf-8 mode
+            encoding = locale.getencoding()
+        else:
+            encoding = locale.getpreferredencoding(False)
         out = subprocess.run(
             ("lsof", "-Ffn0", "-p", str(os.getpid())),
             stdout=subprocess.PIPE,
             stderr=subprocess.DEVNULL,
             check=True,
             text=True,
-            encoding=locale.getpreferredencoding(False),
+            encoding=encoding,
         ).stdout
 
         def isopen(line: str) -> bool:

testing/test_parseopt.py

@@ -290,10 +290,10 @@ class TestParser:
 
 
 def test_argcomplete(pytester: Pytester, monkeypatch: MonkeyPatch) -> None:
-    try:
+    if sys.version_info >= (3, 11):
         # New in Python 3.11, ignores utf-8 mode
-        encoding = locale.getencoding()  # type: ignore[attr-defined]
-    except AttributeError:
+        encoding = locale.getencoding()
+    else:
         encoding = locale.getpreferredencoding(False)
     try:
         bash_version = subprocess.run(