Support Python 3.8-provided importlib.metadata (#585)

The importlib_metadata package is a backport of the importlib.metadata
module from Python 3.8. Fedora (and possibly others) no longer package
importlib_metadata because they ship Python versions which have the
functionality built-in.

Signed-off-by: Scott K Logan <[email protected]>

Author: cottsay

ros2cli/ros2cli/entry_points.py

@@ -16,7 +16,10 @@
 from collections import defaultdict
 import logging
 
-import importlib_metadata
+try:
+    import importlib.metadata as importlib_metadata
+except ModuleNotFoundError:
+    import importlib_metadata
 
 """
 The group name for entry points identifying extension points.

ros2doctor/ros2doctor/api/__init__.py

@@ -16,7 +16,10 @@
 from typing import Set
 from typing import Tuple
 
-import importlib_metadata
+try:
+    import importlib.metadata as importlib_metadata
+except ModuleNotFoundError:
+    import importlib_metadata
 
 from ros2doctor.api.format import doctor_warn