msrest is not needed to run tests

--- barman-release-3.11.1/tests/requirements_dev.txt.orig
+++ barman-release-3.11.1/tests/requirements_dev.txt
@@ -6,6 +6,3 @@
 pytest
 mock
 py
-# msrest is a transitive dependency of azure-storage-blob and
-# is broken on python 2 for versions >= 0.7.0
-msrest<0.7.0