Do not use "imp" (deprecation warnign in Python 3)

The following discussion from !104 (merged) should be addressed:

@clemenci started a discussion:

The module imp has been deprecated in Python 3.3. See https://docs.python.org/release/3.8.2/library/imp.html#imp.load_module

When we drop Python 2 support (even on SLC5), we can revert this MR.

Edited Apr 05, 2020 by Marco Clemencic