`build_ext` fails if `build_py` was not called before

For example in [aesara](https://github.com/aesara-devs/aesara):

```
$ python setup.py build_ext -j12
running build_ext
UPDATING build/lib/aesara/_version.py
error: [Errno 2] No such file or directory: 'build/lib/aesara/_version.py'
```