test/test_mediafile.py::OpusTest::test_read_audio_properties fails with mutagen 1.48.1

On the [builds for the openSUSE Tumbleweed package](https://build.opensuse.org/package/show/openSUSE:Factory/python-mediafile) we have started to get an error on this test with the following message:
```
[ 16s] =================================== FAILURES ===================================
[ 16s] _____________________ OpusTest.test_read_audio_properties ______________________
[ 16s]
[ 16s] self =
[ 16s]
[ 16s] def test_read_audio_properties(self):
[ 16s] mediafile = self._mediafile_fixture("full")
[ 16s] for key, value in self.audio_properties.items():
[ 16s] if isinstance(value, float):
[ 16s] self.assertAlmostEqual(getattr(mediafile, key), value, delta=0.1)
[ 16s] else:
[ 16s] > self.assertEqual(getattr(mediafile, key), value)
[ 16s] E AssertionError: 52392 != 66792
[ 16s]
[ 16s] test/test_mediafile.py:455: AssertionError
[ 16s] =========================== short test summary info ============================
[ 16s] FAILED test/test_mediafile.py::OpusTest::test_read_audio_properties - AssertionError: 52392 != 66792
[ 16s] ============== 1 failed, 480 passed, 104 subtests passed in 1.30s ==============
```

On openSUSE We are using the version 1.48.1 of mutagen, which since version 1.48 provides the bitrate ( https://github.com/quodlibet/mutagen/pull/704 ) and from my reading of the [mediagen code](https://github.com/beetbox/mediafile/blob/faaf7444f1c58d6a39f0ba51dad8ea43a71fdf3a/mediafile/__init__.py#L925) it is being used instead of the one calculated by mutagen based on the file size.

I am going to modify the openSUSE package with the following patch:

```patch
Index: mediafile-0.17.0/test/test_mediafile.py
===================================================================
--- mediafile-0.17.0.orig/test/test_mediafile.py
+++ mediafile-0.17.0/test/test_mediafile.py
@@ -987,7 +987,7 @@ class OpusTest(ReadWriteTestBase, unitte
extension = "opus"
audio_properties = {
"length": 1.0,
- "bitrate": 66792,
+ "bitrate": 52392,
"bitrate_mode": "",
"encoder_info": "",
"encoder_settings": "",

```

Feel free to use it or if you think it should be fixed on mutage I can open a bug there.

Open test/test_mediafile.py and inspect OpusTest.audio_properties, especially the expected bitrate for the full Opus fixture. Run test/test_mediafile.py::OpusTest::test_read_audio_properties with mutagen 1.48.1, then run the broader test suite. Done means the updated expectation reflects the reported bitrate and all tests pass.

Written by the indexing model from the issue text.