build

Add ax argument needed in newer matplotlib #240

Job	Run time
openad (global_ocean.90x40x15)	9m 15s
doc_latex	3m 34s
forward (offline_exf_seaice)	4m 22s
doc_html	2m 7s
forward (global_ocean.cs32x15)	3m 30s
openad-other	0s
forward (tutorial_deep_convection)	2m 39s
forward (aim.5l_cs)	2m 15s
forward (isomip)	2m 45s
forward (global_ocean.90x40x15)	2m 5s
forward (tutorial_plume_on_slope)	1m 33s
forward (tutorial_advection_in_gyre)	1m 23s
forward (hs94.cs-32x32x5)	1m 41s
forward (tutorial_global_oce_biogeo)	1m 39s
forward (tutorial_global_oce_in_p)	1m 19s
forward (tutorial_cfc_offline)	1m 20s
	41m 27s

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