Drop Python 3.6 support
_The documentation is not available anymore as the PR was closed or merged._ I've disabled the `test_dummy_dataset_serialize_s3` tests in the Linux CI to avoid the failures (these tests only fail on Windows in 3.6). These failures are unrelated to this PR's changes, and I would like to address this in a new PR. [This comment](https://github.com/pytorch/audio/issues/2363#issuecomment-1179089175) explains the issue with MP3 decoding in `torchaudio` in the latest release (supports Python 3.7+). I fixed CI by pinning `torchaudio` to `<0.12.0`. Another way to fix this issue is by installing `ffmpeg` with conda or using the unofficial GH action. But I don't think it's worth making CI more complex, considering we can wait for the soundfile release, which should bring MP3 decoding, and drop the `torchaudio` dependency then. Yay for dropping Python 3.6! I think we can merge in this state. Also, if an env has Python version < 3.7 installed, we raise a warning, so I don't think we even need to create (and pin) an issue to notify the contributors of this change.
Remove the fallback imports/checks in the code needed for Python 3.6 and update the requirements/CI files. Also, use Python types for the NumPy dtype wherever possible to avoid deprecation warnings in newer NumPy versions.
