Move ConfigurationLoader to use the PEP-451 style loader

[adamchainz]

Running a project with django-configurations and python -Wall shows this warning:

$ python -Wall manage.py help
<frozen importlib._bootstrap>:918: ImportWarning: ConfigurationLoader.exec_module() not found; falling back to load_module()
...

This is from Python’s frozen importlib reporting that Configurationloader uses the pre-PEP-451 load_module() method, rather than the newer exec_module(). PEP 451 was merged in Python 3.4, so we can safely migrate to exec_module() and prevent that warning.