Remove usage of `eval()` from postprocess.py

### What problem does this PR solve?

The use of `eval()` is a potential security risk. While the use of
`eval()` is guarded and thus not a security risk normally, `assert`s
aren't run if `-O` or `-OO` is passed to the interpreter, and as such
then the guard would not apply. In any case there is no reason to use
`eval()` here at all.

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)
- [x] Other (please describe):

Potential security fix if somehow the passed `modul_name` could be user
controlled.