Remove unused dependency importlib-metadata

[BastianZim]

Description

Insert your PR description here. Thanks for contributing to pyQuil! 🙂

importlib-metadata was only used for python<3.8 but the minimum python is not 3.8, so it is not needed anymore.

Checklist

• The PR targets the master branch
• The above description motivates these changes.
• The change is atomic and can be described by a single commit (your PR will be squashed on merge).
• All changes to code are covered via unit tests.
• Parameters and return values have type hints with PEP 484 syntax.
• Functions and classes have useful Sphinx-style docstrings.
• (New Feature) The docs have been updated accordingly.
• (Bugfix) The associated issue is referenced above using auto-close keywords.