PYADPT-89 - "Make `native_pep` / `_pEp` a submodule of `pEp`"
which is probably not what we want.
If we decide to fix it, the tests' assertions need to be changed.
Since the identities are not being tested in the message tests,
facilitate to create them.
so that they all can run with just one pytest command, they can be
integrated with python test command and with tox and they can be
run separately with different python versions.