build(deps): bump slang from `6e18236` to `d56a398` #394

Job	Run time
Build with Pip (windows-latest, 3.11)	8m 47s
Build with Pip (windows-latest, pypy-3.9)	9m 37s
Build with Pip (macos-latest, 3.11)	3m 9s
Build with Pip (macos-latest, pypy-3.9)	3m 33s
Build with Pip (ubuntu-latest, 3.11)	6m 41s
Build with Pip (ubuntu-latest, pypy-3.9)	6m 47s
	38m 34s

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