Announce plans for changing default packet manipulation module to bf_pktpy

[jafingerhut]

Feel free to suggest alternative steps, if these are lacking:

• Announcement:
  • Add near top of README.md the plans to make a backwards-incompatible change within X months from the announcement, which is: changing the default packet manipulation module from scapy to bf_pktpy
  • include a link to instructions on how to select scapy or bf_pktpy as the desired packet manipulation module.
  • If one of the following two lines of code are reached, consider printing a warning that the default will change in release 1.0:
    • https://github.com/p4lang/ptf/blob/main/ptf#L450
    • https://github.com/p4lang/ptf/blob/main/src/ptf/packet.py#L35
• Soon before the next step, make a release to PyPI of the current latest version of ptf, with only a minor version increment from the latest release.
• Simultaneously do these things:
  • change the default packet manipulation module to bf_pktpy
  • increase the major version number
  • make a release with those changes with the new major version number.

Some discussion about reasons for this: p4lang/p4c#5145 (comment)