Installation of Pandas in Pip v10.0.0b2 on PyPy fails
#### Code Sample, a copy-pastable example if possible

```bash
root@9b34418ce20d:/# /opt/pypyenv/bin/pip install pandas
Looking in indexes: https://devpi.myindex/root/myindex/+simple/
Collecting pandas
  Downloading https://devpi.myindex/root/pypi/+f/c7a/2757b60774825/pandas-0.22.0.tar.gz (11.3MB)
    100% |################################| 11.3MB 1.5MB/s 
  Could not find a version that satisfies the requirement Cython (from versions: )
No matching distribution found for Cython
```
#### Problem description

See https://github.com/pypa/pip/issues/5171 for the other side of this issue. Pandas defines a `pyproject.toml` for PEP 518 compliance. Unfortunately, Pip 10 only partially supports PEP 518; it requires that all dependencies be available as binary distributions which is generally not the case for implementations like PyPy. As such installation fails as above.
