Update CI jobs xcode/macOS version to match PyTorch core

[mthrok]

xcode Version 9 is no longer supported by Apple. https://developer.apple.com/support/xcode/

Updating it to
xcode: 12.0
macOS: 10.15

See https://circleci.com/docs/2.0/testing-ios/#supported-xcode-versions

See also: pytorch/audio#1485

[datumbox]

LGTM.

Our CI is broken, so to confirm that this change fixes at least some of the issues, I did some digging. On latest master, the following macOS jobs fail:

• unittest_macos_cpu_py3.6
• unittest_macos_cpu_py3.7
• unittest_macos_cpu_py3.8
• unittest_macos_cpu_py3.9

in this PR all the above methods seems to run fine. Let's wait until all the CI tests complete to confirm and then merge.

Edit: The tests still fail but that's because of an issue on a unit-test. It's unrelated and we'll ping the author of the PR who broke it to investigate.

[datumbox]

@fmassa @bjuncek Now that the CI can run properly, it uncovered an issue with the following unit-test:

    @unittest.skipIf(av is None, "PyAV unavailable")
    def test_frame_reading(self):
        for test_video, config in test_videos.items():
            full_path = os.path.join(VIDEO_DIR, test_video)
    
            av_reader = av.open(full_path)
    
            if av_reader.streams.video:
                video_reader = VideoReader(full_path, "video")
                for av_frame in av_reader.decode(av_reader.streams.video[0]):
                    vr_frame = next(video_reader)
    
                    self.assertAlmostEqual(
                        float(av_frame.pts * av_frame.time_base),
                        vr_frame["pts"],
                        delta=0.1,
                    )
    
                    av_array = torch.tensor(av_frame.to_rgb().to_ndarray()).permute(
                        2, 0, 1
                    )
                    vr_array = vr_frame["data"]
                    mean_delta = torch.mean(
                        torch.abs(av_array.float() - vr_array.float())
                    )
                    # on average the difference is very small and caused
                    # by decoding (around 1%)
                    # TODO: asses empirically how to set this? atm it's 1%
                    # averaged over all frames
>                   self.assertTrue(mean_delta.item() < 2.5)
E                   AssertionError: False is not true

test/test_videoapi.py:97: AssertionError

[prabhat00155]

Thanks @mthrok!