Replace deprecated imp module with importlib

[hugovk]

This project uses the imp module which has been deprecated since Python 3.4 and set for removal in 3.12:

• Raised PendingDeprecationWarning since 3.4 (2014)
• Raised DeprecationWarning since 3.5 (2015)
• Updated DeprecationWarning to say removal in 3.12 since 3.10 (2021)
• Removal planned for 3.12 (2023)

Python 3.12 is set for release on 2023-10-02 and this library is one of the top 5,000 most-downloaded from PyPI.

Please could you upgrade to use importlib? The imp docs have suggestions on what to use to replace each function and constant.

[mgorny]

Ping. This is blocking Python 3.12 support on flask-nav.

[mgorny]

Ping.