remove walrus operator to support Python 3.7 #91

Job	Run time
nix / nix-build	28s
docs / build-and-push-docs (shivupa/cclib-ci:py311-edge)	53s
unit-tests / run-tests (shivupa/cclib-ci:py38-edge)	1m 29s
unit-tests / run-tests (shivupa/cclib-ci:py39-edge)	1m 29s
unit-tests / run-tests (shivupa/cclib-ci:py310-edge)	1m 31s
unit-tests / run-tests (shivupa/cclib-ci:py311-edge)	1m 29s
unit-tests / check-installation (wheel, 3.7)	21s
unit-tests / check-installation (wheel, 3.8)	19s
unit-tests / check-installation (wheel, 3.9)	15s
unit-tests / check-installation (wheel, 3.10)	18s
unit-tests / check-installation (wheel, 3.11)	23s
unit-tests / check-installation (wheel, 3.12)	22s
unit-tests / check-installation (source, 3.7)	26s
unit-tests / check-installation (source, 3.8)	17s
unit-tests / check-installation (source, 3.9)	21s
unit-tests / check-installation (source, 3.10)	25s
unit-tests / check-installation (source, 3.11)	25s
unit-tests / check-installation (source, 3.12)	27s
publish / Sign the Python distribution with Sigstore and upload them to GitHub Release	0s
publish / Publish Python distribution to PyPI	0s
publish / Publish Python distribution to TestPyPI	0s
all	0s
	11m 38s

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