Remove dependency on p4p fork

[Monarda]

Currently there are two reasons we are dependent on our p4p fork.

1. add support to initialise NTEnum like NTScalar #154 which makes NTEnum initialisation easier. This PR has been accepted but not in time to make the latest release (4.2.1).
2. New methods for Handler and SharedPV is required to implement our method for handling Normative Type logic.

In both cases I have followed the same strategy to resolve the problem. I have created p4p_ext equivalents for the modules and then used monkey patching to implement the small necessary changes to merge the PRs into the p4p library code.

This means that to use the patched code you use p4p_ext.server.thread.SharedPV or p4p_ext.nt.NTENum instead of p4p.server.thread.SharedPV or p4p.nt.NTEnum respectively. The p4p_ext code has been patched to use the monkey patched versions. In the case of p4p_ext.nt.NTENum it should automatically disable itself at the next version increment of p4p.version.

I have also modified the pyproject.yaml and builds so that they now use plain p4p.

[kurup]

I've looked at the changes and I'm happy for this to be merged.