Ignore DeprecationWarnings to fix tests on py3.11

Pass `-Wignore::DeprecationWarning` when spawning autopep8 in tests
to fix the two test failures when running on Python 3.11:

```
======================================================================
FAIL: test_in_place_no_modifications_no_writes (__main__.CommandLineTests.test_in_place_no_modifications_no_writes)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/tmp/autopep8/test/test_autopep8.py", line 5518, in test_in_place_no_modifications_no_writes
    self.assertEqual(err, b'')
AssertionError: b'/tmp/autopep8/autopep8.py:182: Deprecatio[137 chars]ze\n' != b''

======================================================================
FAIL: test_in_place_no_modifications_no_writes_with_empty_file (__main__.CommandLineTests.test_in_place_no_modifications_no_writes_with_empty_file)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/tmp/autopep8/test/test_autopep8.py", line 5530, in test_in_place_no_modifications_no_writes_with_empty_file
    self.assertEqual(err, b'')
AssertionError: b'/tmp/autopep8/autopep8.py:182: Deprecatio[137 chars]ze\n' != b''
```

This is a temporary fix until issue #581 is resolved.

test/test_autopep8.py

@@ -40,13 +40,14 @@
 )
 
 if 'AUTOPEP8_COVERAGE' in os.environ and int(os.environ['AUTOPEP8_COVERAGE']):
-    AUTOPEP8_CMD_TUPLE = ('coverage', 'run', '--branch', '--parallel',
+    AUTOPEP8_CMD_TUPLE = (sys.executable, '-Wignore::DeprecationWarning',
+                          '-m', 'coverage', 'run', '--branch', '--parallel',
                           '--omit=*/site-packages/*',
                           os.path.join(ROOT_DIR, 'autopep8.py'),)
 else:
     # We need to specify the executable to make sure the correct Python
     # interpreter gets used.
-    AUTOPEP8_CMD_TUPLE = (sys.executable,
+    AUTOPEP8_CMD_TUPLE = (sys.executable, '-Wignore::DeprecationWarning',
                           os.path.join(ROOT_DIR,
                                        'autopep8.py'),)  # pragma: no cover