#122: Add typing-extensions as dependency when python_version < 3.8

[amacf]

Fixes #122: When importing platformdirs on Python < 3.8, typing_extensions might not be installed. Which would cause an import error.

This adds typing-extensions as a dependency when using < python3.8

[gaborbernat]

Let's keep it as is, but add typing extension as a dependency for older python versions.

[Julian]

CI passed on the release (including on 3.7), so someone should look at why that is as well probably.

[amacf]

typing-extensions is a dependency of the package importlib-metadata, which is a dependency of pytest. So the unit tests pass because typing-extensions happens to be installed by the testing library.