Use importlib (Python 3.12 compatible)

[Kilo59]

Replace imp usage with importlib imp has been removed in Python 3.12 so this should make this package compatible.

• https://github.com/syrusakbary/snapshottest/issues/142#issuecomment-703166275

Tests all pass locally on Python 3.12.

• This implementation is much simpler than #168

[RobinTail]

Fixes #166