Audiofile

* Changed: use ``ffmpeg`` or ``mediainfo``
if ``sox`` binary is missing
instead of raising an error
* Changed: improve ``FileNotFoundError`` message
when ``ffmpeg`` or ``mediainfo`` are required,
but not installed
* Fixed: always raise ``RuntimeError``
for broken files or missing files
* Fixed: mention expected ``FileNotFoundError``
when ``ffmpeg`` or ``mediainfo`` are required,
but not installed
* Fixed: use ``soundfile``
for converting WAV, FLAC, OGG files
with ``audiofile.convert_to_wav()``

* Removed: Python 3.6 support

* Fixed: raise ``RuntimeError``
for broken or empty non SND files

* Fixed: ``audiofile.duration()`` returns now ``0.0``
for an empty WAV file with wrong file extension
instead of ``None``

* Added: support for Python 3.9
* Added: tests for Windows
* Added: tests for macOS
* Added: ``sloppy`` argument to ``audiofile.duration()``
for faster processing of non SND formats
* Changed: ``audiofile.write()`` raises ``RuntimeError``
instead of ``SystemExit``
* Changed: added ``sloppy=True`` results
when accessing metadata of MP3 and MP4 files in the benchmarks
* Removed: deprecated precision argument from ``audiofile.write()``

* Fixed: make ``dtype`` keyword argument available in ``audiofile.read()``