[backends] Fix PyAV 19+ compatibility

[Breakthrough]

It seems like a few values now return av.Rational now, similar to https://ffmpeg.org/doxygen/4.0/group__lavu__math__rational.html - for compatibility we convert back to Fraction internally, but it represents the same thing effectively. From the changelog:

Rational attributes (time_base, average_rate, base_rate, guessed_rate, framerate, rate, sample_aspect_ratio, and display_aspect_ratio) now return av.AVRational rather than fractions.Fraction, and are never None: an unset value is the falsy AVRational(0, 1). Test them with if not stream.time_base: instead of is None. Setters still accept a fractions.Fraction