Allow setting the max_bit_rate and min_bit_rate in codec context

[hmaarrfk]

These variables are documented as part of ffmpeg.

It might be useful for others to read and write them.

[jlaine]

OK could we have a test for this please?

The CI failure should go away if you rebase on top of the latest code.

[hmaarrfk]

I'm actually no longer sure what should happen when these things change. I had to specificy the codec context options dictionary to have a visible change in my output quality.

[jlaine]

I'm guessing this PR hasn't been rebased yet as the new tests are against pypy3.9 and not pypy3?

[hmaarrfk]

I've rebased the patch, but I'm not sure what effect "writing" to these two properties has.

[WyattBlue]

This PR is too old.