"develop" installation broken

Attempting to pip install editable (/develop) yields broken executables. For dl-serve::
```
Traceback (most recent call last):
File "/usr/local/bin/dl-serve", line 10, in
import dripline
File "/usr/local/src_py/dripline/__init__.py", line 1, in
from . import core
File "/usr/local/src_py/dripline/core/__init__.py", line 4, in
from _dripline.core import *
ModuleNotFoundError: No module named '_dripline.core'
```

Reproducible by changing this line to `RUN pip install -v -e /usr/local/src_py` in Dockerfile:
https://github.com/driplineorg/dripline-python/blob/develop/Dockerfile#L26

Develop installation of the code is very useful for rapidly developing and testing modifications to the dl-py code. Would be great to get this feature fixed - probably it's an issue coming from the pybind11.