pip.main no longer public api

[This code](https://github.com/mrakitin/bnlcrl/blob/04729e86cce81d143d9b3e824cc82db8e3751c80/setup.py#L11)
will not work with pip 10.x.

pykern has been updated, but there is no easy fix for doing this trick any more. Rather, you must install pykern before bnlcrl. The following is the recommended way to install from pip:

```sh
pip install -r requirements.txt -e .
```

