Xenial: update python package

imgtool 1.6.0 doesn't exist, lets install a working version.

Change-Id: Id2539cb29c42868a99b1dd4c215e684bbad9b321

Signed-off-by: Benjamin Copeland <ben.copeland@linaro.org>
Change-Id: I850e0af156e854ed3a2a66d6ecc7c9ccc0d13bab
diff --git a/xenial-amd64-tf-m-build/requirements_python3.txt b/xenial-amd64-tf-m-build/requirements_python3.txt
index 507db57..3520cc8 100644
--- a/xenial-amd64-tf-m-build/requirements_python3.txt
+++ b/xenial-amd64-tf-m-build/requirements_python3.txt
@@ -10,4 +10,4 @@
 sphinx-rtd-theme==0.4.3
 sphinxcontrib-plantuml==0.15
 xmltodict==0.12.0
-imgtool==1.6.0
+imgtool==1.5.0