[external/FFmpegFD] Fix ffmpeg detection with --ffmpeg-location ... (etc)

[dirkf]

Boilerplate: own code/bug fix

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• Bug fix
• Improvement
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Description of your pull request and other information

The ffmpeg downloader FFmpegFD depends on the corresponding post-processor FFmpegPostProcessor. The availability of a downloader is a class property, while the availability of a post-processor is an instance property. Due to a combination of code issues, a viable ffmpeg installation might not be found for downloading when a custom location was specified with --ffmpeg-location ... or its API equivalent, as in #32735.

This PR fixes the issue by splitting the availability computation:

• the availability of the downloader as a class property is made to depend on the existence of the FFmpegPostProcessor class (normally guaranteed)
• the actual availability at download time depends on the availability computed by the post-processor object created for the download.

Previously the first step would always fail with a custom ffmpeg location not on the PATH; even if it had succeeded, the instance-based availability check would fail because the associated post-processor was constructed using the object itself rather than its parent YoutubeDL.

Summary of changes:

• a new compat feature compat_contextlib_suppress is added to allow the with suppress(exceptions) ... syntax to be used across Py2/3,. and applied in the utils module
• also, compat_subprocess_Popen is added so that Popen() is a context manager across Py2/3
• with these, a long-standing mystery "Resource Warning" diagnostic in the external downloader test is removed
• ffmpeg detection by FFmpegPostProcessor is simplified
• the availability logic in FFmpegFD is reworked as above with a more relevant error message.