Lock Python version for node-gyp #79

Job	Run time
build-wasm	3m 23s
test-native (ubuntu-latest, 18.x)	1m 20s
test-native (ubuntu-latest, 20.x)	1m 22s
test-native (ubuntu-latest, 21.x)	1m 19s
test-native (macOS-latest, 18.x)	2m 26s
test-native (macOS-latest, 20.x)	3m 7s
test-native (macOS-latest, 21.x)	2m 54s
test-native (windows-latest, 18.x)	4m 36s
test-native (windows-latest, 20.x)	6m 12s
test-native (windows-latest, 21.x)	3m 53s
build-web	6s
test-wasm (ubuntu-latest, 18.x)	3m 49s
test-wasm (ubuntu-latest, 20.x)	8m 16s
test-wasm (ubuntu-latest, 21.x)	6m 44s
test-wasm (macOS-latest, 18.x)	7m 5s
test-wasm (macOS-latest, 20.x)	7m 18s
test-wasm (macOS-latest, 21.x)	7m 8s
test-wasm (windows-latest, 18.x)	7m 46s
test-wasm (windows-latest, 20.x)	14m 51s
test-wasm (windows-latest, 21.x)	11m 2s
test-web (deno, ubuntu-latest)	5m 47s
test-web (deno, macOS-latest)	5m 28s
test-web (deno, windows-latest)	12m 28s
test-web (chrome, ubuntu-latest)	4m 35s
test-web (chrome, macOS-latest)	5m 33s
test-web (chrome, windows-latest)	8m 58s
	2h 27m 26s

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