Add support r_rate from ffmpeg r_frame_rate

[KengoSawa2]

Hi Thank you great library!!

I am creating a tool to excel metadata with PyAV.

Current PyAV can refer to avg_frame_rate.
However, when displayed with avg_frame_rate, it shows a value different from popular players.
What I confirmed are Mac's QT player, VLC, djv, mediainfo.

Another problem.
When trying to display "TC" by passing avg_frame_rate to the TimeCode library, it tends to be a strange value and it is difficult to use.

For the above two reasons, I think it would be more convenient to make it possible to access r_frame_rate with PyAV.

#319 may be somewhat related(reason is different from me)

[mikeboers]

I'm considering renaming this attribute as far as Python is concerned.

avg_frame_rate seems reasonable as-is.

Perhaps this should be base_frame_rate and av_guess_frame_rate(...) should be exposed as best_frame_rate or guessed_frame_rate

[mikeboers]

Gonna keep this going in #564