Add bitstream filters

[skeskinen]

Loosely based on #565
Additional discussion in #489

This adds bindings for AVBSFContext (BitStreamFilterContext)

I decided against adding binding for AVBitStreamFilter, because it seems like a little bit useless API. You can't do anything with such object besides creating a new BSF context. (except checking which codecs are supported for that BSF, I guess)
It makes the code simpler and imo easier to maintain. Let me know if this is a mistake and those bindings should be added as well.

I think it makes sense to force BitstreamFilterContext creation through the string parsing API, since that's the only way I can see how to set filter options and also allows things like filter chains, etc.

Adds tests for 'chomp', 'setts' and 'h264_mp4toannexb' filters. We could also test that filter chains work.

h264_mp4toannexb requires that AVCodecParameters *par_in is set correctly. To that end BitStreamFilterContext takes a Stream as an argument and copies the codec parameters from there.

There are also attributes time_base_in, time_base_out which are probably required for 100% support of setts option constraints. Let me know if those should be hooked up as well. I guess it would require that BitStreamFilterContext takes separate in_stream, out_stream arguments instead of single stream.

[skeskinen]

Excuse me about the linter stuff. I can't run make lint locally because the flake8 always fails. I think it should work now. Both black and isort are happy

[WyattBlue]

I think the PR is good as it is now.
One thing I changed was inlining def self._recv and def self._send to cpdef filter. This eliminates the overhead in calling Python functions and IMO makes it easier to read. I also add type hints and docstrings.

I'll give you a chance to comment, then it can be merged.

[skeskinen]

Looks good to me. Thanks!