Upgrading matplotlib to last stable version to support pytho 3.8 used on the test setups

Change-Id: I6cb6a35f07288f39c551983e6d1d4b189d6b2882
diff --git a/requirements.txt b/requirements.txt
index b53f166..f51b693 100755
--- a/requirements.txt
+++ b/requirements.txt
@@ -11,7 +11,7 @@
 git+https://github.com/zdw/robotframework-importresource@b81b87aabaee0594e966687b41e3674b866f28ee
 cord-robot==2.1.4
 virtualenv
-matplotlib==3.0.3
+matplotlib>=3.0.3
 requests==2.24.0
 grpc-robot>=2.7.0
 kafka-robot>=2.3.2