[chromperf] Update chart_handler to allow python 3.
[chromperf] Update chart_handler to allow python 3.

Bug: chromium:1298177
Change-Id: I13e8a670da3cfea6674e9b6f0de1cf1122f07456
Reviewed-on: https://chromium-review.googlesource.com/c/catapult/+/3984121
Commit-Queue: Wenbin Zhang <45ad8a1ff616a38a2202fd35571df81897c5f14f@google.com>
Reviewed-by: Leina Sun <4dc7485e04f6a3a1d05495d5781efc53aef710e0@google.com>

catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult