dev-python/pypy: Inline deps and include dev-python/pycparser dep

dev-python/pypy: Inline deps and include dev-python/pycparser dep

Replace the autogenerated || dependency with an inlined version,
and include dev-python/pycparser in it when using CPython 2.7 (it is
built-in in dev-python/pypy, through CFFI). Enforce the dependency
only when USE=-low-memory, since otherwise CPython is not used, and we
can't enforce the dependency on PyPy anyway as it would cause circular
deps.

Closes: https://bugs.gentoo.org/634306