pybind11 runtime dependency for CPPE?

In an environment where `CPPE` is installed, "`pip check`" fails unless `pybind11` is also installed:

```
$ pip check
cppe 0.3.1 requires pybind11, which is not installed.

$ echo $?
1
```

Is `pybind11` really a runtime dependency of CPPE, or is it only required when building/installing CPPE?