Merge pull request #785 from igraph/feat/mean_degree #1600

Job	Run time
Build wheels on Linux (aarch64/musllinux)	0s
Build wheels on Linux (aarch64/manylinux)	0s
Build sdist and test extra dependencies	2m 38s
Build wheels on Linux (x86_64)	21m 47s
Build wheels on Linux (i686)	17m 57s
Build wheels on macOS (x86_64)	20m 39s
Build wheels for WebAssembly	7m 3s
Build wheels on Windows (Win32)	11m 45s
Build wheels on macOS (arm64)	10m 6s
Build wheels on Windows (x64)	27m 1s
Build with sanitizers for debugging purposes	39s
	1h 59m 35s

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