Imageio-ffmpeg

* Support for Aarch64.
* Basic support to include audio when writing videos.

* Support for Python 3.9.
* File object is closes after use just in case.

* Allow the `ffmpeg_timeout` arg in `write_frames()` to be `None`.
* Don't prevent sigint propagation for ffmpeg call in `count_frames_and_secs()`.
* Removed a line in the part where `read_frames()` signals ffmpeg to quit, preventing "Invalid Argument" (Windows) and "Broken Pipe" (Linux) warnings. Previously we did not see these warnings because they were swallowed pre v0.4.0.
* More tests to avoid regressions.

This release includes several improvements and fixes to long-standing issues. We strongly recommend to upgrade.

Changes to the API:

* The default value for `ffmpeg_timeout` is now zero, e.g. just wait for ffmpeg.
* Added bits_per_pixel parameter to read_frames.

Fixes:

* Improved handling of interrupts during reading/writing/closing-down.
* Fixed that keyboard interrups would propagate to ffmpeg.
* Fixed that ffmpeg would sometimes hang after reading.
* Fixed the occurance of segfaults under certain conditions.
* Stopped executable window from flashing on Windows.

Other improvements:

* Support for Cygwin.
* Official support for Python 3.8.
* Support for pathlib.Path filenames.

* Use pkg_resources to get binary.
* Include readme and license in sdist
* Add 'ffmpeg_timeout' flag to write_frames (14)
* Tweaks to killing ffmpeg on writing (15)