mrw: bump tools rev and switch to python3

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I50543a2948dd6a43e93462bd32c774c216702e06
diff --git a/classes/mrw-rev.bbclass b/classes/mrw-rev.bbclass
index b0a6b1f..2938430 100644
--- a/classes/mrw-rev.bbclass
+++ b/classes/mrw-rev.bbclass
@@ -2,4 +2,4 @@
 MRW_API_SRCREV ?= "60c8e10cbb11768cd1ba394b35cb1d6627efec42"
 
 MRW_TOOLS_SRC_URI ?= "git://github.com/openbmc/phosphor-mrw-tools"
-MRW_TOOLS_SRCREV ?= "715c97ea76bb6c976e57dfa899f76a7106a7c2d5"
+MRW_TOOLS_SRCREV ?= "721dcbd23da4f9679ddf58119c478922bf420a1e"