Wrong dtype for codec_context.flags #1718

I just noticed that both flag fields (flags, flags2) in the codec context are type long in C but they should be unsigned long. The current setup leads to some strange errors when naively setting, for example, the CLOSED_GOP flag ... could we change this?

...
stream = container.add_stream("h264", 60)
stream.flags |= 0x80000000  # set cgop flag

results in

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "av\\codec\\context.pyx", line 119, in av.codec.context.CodecContext.flags.__set__
OverflowError: Python int too large to convert to C long

The current way to set this flag (in v14.0.1) is to use the corresponding signed representation of the flag. This is slightly unintuitive and not very self-explanatory:

stream = container.add_stream("h264", 60)
stream.flags |= -2147483648  # set cgop flag