Bump python from `7476637` to `3966b81` #2766

Job	Run time
k8s-tests-orig-vs-opt-comparison	1m 47s
lint	13s
test-docker	13s
calico-tests	28s
test-docker-ubi	34s
unit-tests	12s
build_package	22s
calico-tests-orig-vs-opt-comparison	36s
fw-rules-assertion-tests	29s
istio-tests	50s
istio-tests-orig-vs-opt-comparison	1m 8s
k8s-tests	1m 15s
test_package	10s
	8m 17s

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