Fix frame number generation in pyav_reader._gen_frames

[Britefury]

pyav_reader._gen_frames sometimes yields frame objects with incorrect frame numbers due to incorrect rounding.
Fixed by using round rather than int.
This has resulted in PyAVTimedReader.get_frame returning frame objects whose frame_no field has a different frame than the value passed to get_frame.
I was able to confirm this on a video file and verify that PyAVTimedReader and PyAVIndexedReader return different frames when randomly seeking. Furthermore, converting the video to a sequence of PNGs using ffmpeg and reading those agrees with PyAVIndexedReader but not PyAVTimedReader. This fix addresses this, resulting in PyAVTimedReader returning frame consistency with the others.
I may be able to release the video file for testing, but will have to check.

[Britefury]

Re-based onto master (as of 2021/Mar/10) and fixed bugs that were preventing tests from passing.
(the fix was to wrap the round call in an int type cast).

[nkeim]

Thank you @Britefury for tracking down this edge case, and thanks for the rebase!