install both Python2 and Python3 #998

Job	Run time
bundled_gdal_coverage	5m 16s
test_distros_shared (ubuntu:22.04)	3m 28s
test_asan	4m 6s
test_distros_shared (ubuntu:20.04)	3m 46s
test_distros_bundled (amazonlinux:2022)	15m 35s
test_distros_shared (debian:bullseye)	3m 58s
test_mac (16.x)	7m 51s
test_distros_shared (fedora:37)	6m 31s
test_mac (18.x)	7m 54s
test_distros_shared (centos:stream8)	48m 59s
test_mac (20.x)	7m 13s
test_mac_debug	8m 33s
test_mac (21.x)	7m 17s
test_node (16, shared)	3m 58s
test_node (16, bundled)	4m 5s
test_node (18, shared)	3m 55s
test_win (16.x)	18m 11s
test_node (18, bundled)	4m 18s
test_win_debug	16m 6s
test_win (18.x)	21m 49s
test_node (20, shared)	3m 35s
test_win (20.x)	19m 30s
test_node (20, bundled)	3m 35s
test_win (21.x)	19m 20s
test_node (21, shared)	4m 5s
test_node (21, bundled)	3m 59s
	4h 16m 53s

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