Fix compatibility with PyPy #208

Job	Run time
tests / Load tox environments	7s
tests / py38-test-casa-oldestdeps (ubuntu-latest)	3m 25s
tests / py38-test-casa (ubuntu-latest)	4m 10s
tests / py39-test (ubuntu-latest)	2m 14s
tests / py310-test (ubuntu-latest)	2m 35s
tests / py311-test-devdeps (ubuntu-latest)	2m 30s
tests / py38-test-casa (macos-latest)	3m 59s
tests / py39-test (macos-latest)	3m 59s
tests / py310-test (macos-latest)	4m 37s
tests / py311-test-devdeps (macos-latest)	3m 38s
tests / py38-test (windows-latest)	4m 2s
tests / py39-test (windows-latest)	4m 52s
tests / py310-test (windows-latest)	4m 28s
tests / py311-test-devdeps (windows-latest)	4m 5s
	48m 41s

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