requirements: bump sigstore-python #269

Job	Run time
selftest (ubuntu-latest)	19s
selftest-staging	18s
selftest (macos-latest)	36s
selftest-xfail-invalid-inputs (--this-should-not-work)	17s
selftest (windows-latest)	1m 6s
selftest-xfail-invalid-inputs (/tmp/extremely-nonexistent-file)	14s
selftest-glob	18s
selftest-glob-multiple	19s
selftest-custom-paths	16s
selftest-upload-artifacts	23s
selftest-verify	18s
selftest-xfail-glob-input-expansion	15s
selftest-identity-token	19s
selftest-xfail-verify-missing-options (true)	15s
selftest-xfail-verify-missing-options (true, https://token.actions.githubusercontent.com)	16s
selftest-xfail-verify-missing-options (true, https://github.com/sigstore/gh-action-sigstore-pytho...	15s
selftest-xfail-verify-missing-options (false, https://token.actions.githubusercontent.com)	16s
selftest-xfail-verify-missing-options (false, https://github.com/sigstore/gh-action-sigstore-pyth...	15s
all-selftests-pass	2s
	6m 17s

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