use swap{16,32,64} from <endian.h> on OpenBSD

[timkuijsten]

This fixes a compile issue on OpenBSD, because bswap is not supported. Tested on OpenBSD 6.8 stable.

Furthermore, the man pages of NetBSD and FreeBSD indicate these platforms do support bswap{16,32,64}.

Same as Chia-Network/chiapos#183.

[timkuijsten]

Note regarding the "Build wheel on macos-latest " errors. I can build on macOS 11.2.3 using pip wheel .:

Building wheels for collected packages: chiavdf
  Building wheel for chiavdf (PEP 517) ... done
  Created wheel for chiavdf: filename=chiavdf-1.0.2.dev1-cp39-cp39-macosx_11_0_x86_64.whl size=962835 sha256=b4d741c1b6915889ef64fb9808dc684bc799baba4d744f90d219fd7334125feb
  Stored in directory: /private/var/folders/0l/r8yzvdkj16g1gsb61g9jrvbw0000gn/T/pip-ephem-wheel-cache-_8mlr__d/wheels/67/9a/05/7c8dd1fbd1b84e618112fef0afc9f3f2d267add980d1b4842b
Successfully built chiavdf