Replace deprecated imp module with importlib

[Hugo]

This project's tests use the [`imp` module](https://docs.python.org/3/library/imp.html) which has been deprecated since Python 3.4 and removed in 3.12:

* Raised `PendingDeprecationWarning` since [3.4 (2014)](https://github.com/python/cpython/commit/e4f41deccf94ccc798b1eb1f44657ade66669a60)
* Raised `DeprecationWarning` since [3.5 (2015)](https://github.com/python/cpython/commit/c0d91aff9a3b91307b26e8b7c34dfbf27bbdd43a)
* Updated `DeprecationWarning` to say removal in 3.12 since [3.10 (2021)](https://github.com/python/cpython/commit/dc6d3e1e4c0c1e4b2210edab8fb4762569dc2936)
* Removal planned for [3.12](https://github.com/python/cpython/issues/98040) [(2023)](https://github.com/python/cpython/pull/98573)

Python 3.12 is [set for release on 2023-10-02](https://devguide.python.org/versions/) and this library is one of the [top 5,000 most-downloaded from PyPI](https://hugovk.github.io/top-pypi-packages/).

Please could you upgrade to use `importlib`? The [`imp` docs](https://docs.python.org/3.11/library/imp.html) have suggestions on what to use to replace each function and constant.

[Alexandre Detiste (alexandre-detiste)]

please don't use python3-zombie-imp ...

~~A particularly mischevious act of necromancy. That is,~~