Move to importlib.metadata from pkg_resources

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To load entry_points the new technique is importlib.metadata as outlined here: https://docs.python.org/3/library/importlib.metadata.html

This will need to be conditionally done for 3.8 or later or else development forked from older systems w/o python 3.8.

When this is done. Unpin the CI.

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Related changes for reference doing backwards compatiability to pre 3.8

• https://github.com/colcon/colcon-core/blob/291f16c9c0bd646f541eaa80f0875f77b83bdc25/setup.cfg#L33
• https://github.com/PyCQA/flake8-import-order/pull/203/files