Remove "importlib_metadata" compatibility layer

At the moment, the codebase relies on the external package `importlib_metadata` but starting Python 3.10, we will be able to rely on `importlib.metadata` in the standard library. When we can drop support for Python < 3.10, we can remove the compatibility layer which tries to import the external `importlib_metadata` and falls back to standard library `importlib.metadata` if it doesn't exist.

Additionally, compatibility code around the presence of the `select` interface on entry point objects can also be removed when we drop support for Python < 3.10.