Enable subclassing of RoutingCost from python #228

Job	Run time
formatting	1m 14s
lint	28m 40s
test (noetic)	15m 20s
test (galactic)	14m 14s
test (humble)	8m 34s
full_conan_installation (3.8)	13m 24s
full_conan_installation (3.9)	13m 27s
full_conan_installation (3.10)	12m 40s
full_conan_installation (3.11)	12m 47s
full_conan_installation (3.12, manylinux_2_31_x86_64, conanio/gcc10:latest)	12m 52s
	2h 13m 12s

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