pyupgrade to 3.9

[gadomski]

Related Issue(s):

• Closes Run pyupgrade to get to 3.9 #1235

Description:

Applies pyupgrade, and installs a pre-commit rachet to make sure we don't backslide. I don't use a local pre-commit version (like the other hooks) b/c users shouldn't have to have it installed in their environment IMO.

No functional changes, and it's only formatting so no CHANGELOG needed.

PR Checklist:

• pre-commit hooks pass locally
• Tests pass (run scripts/test)
• Documentation has been updated to reflect changes, if applicable
• This PR maintains or improves overall codebase code coverage.
• Changes are added to the CHANGELOG. See the docs for information about adding to the changelog.

[codecov]

Codecov Report

Attention: 36 lines in your changes are missing coverage. Please review.

Files	Coverage Δ
pystac/__init__.py	94.64% <100.00%> (ø)
pystac/asset.py	97.14% <100.00%> (ø)
pystac/cache.py	92.80% <100.00%> (ø)
pystac/collection.py	93.07% <100.00%> (+0.02%)	⬆️
pystac/common_metadata.py	92.79% <100.00%> (ø)
pystac/errors.py	100.00% <100.00%> (ø)
pystac/extensions/base.py	88.88% <100.00%> (+0.12%)	⬆️
pystac/extensions/classification.py	95.07% <100.00%> (+0.03%)	⬆️
pystac/extensions/ext.py	96.53% <100.00%> (ø)
pystac/extensions/file.py	91.91% <100.00%> (+0.05%)	⬆️
... and 37 more

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