Cython language_level Warning - switch to Python3?
When compiling with Cython the following warning gets issued:

```bash
Cython/Compiler/Main.py:367: FutureWarning: Cython directive 'language_level' not set, using 2 for now (Py2). This will change in a later release! File: clones/pandas/pandas/_libs/groupby.pxd
```

Haven't looked into the implications as of yet but probably worth switching to Python3, especially given the upcoming drop of Python2 support