Test PyPy isn't newer than CPython

[jakirkham]

If PyPy becomes newer than CPython for a specific version (like 3.8), Conda will default to installing PyPy everywhere (instead of CPython as intended). Would be good to add a test that PyPy isn't newer that CPython to avoid this.

Better still would be to avoid this behavior in or from solvers somehow

cc @mattip ( who raised this in comment https://github.com/conda-forge/pypy-meta-feedstock/pull/24#issuecomment-1412676270 )

[mattip]

It seems something like this might work

conda search python={{ python_version }} | grep cpython
if [[ $? -eq 1 ]]; then 
    echo cannot release pypy without a corresponding cpython;
    exit -1
fi