WIP: deps: bump test PyYAML requirement to 6.0

In this patch we bump PyYAML requirement since version 6.0 has
various vulnerability fixes (for example, [1-3]). Since PyYAML is used
only in tests, the only reason of this patch is to remove Dependabot
alerts.

1. msgpack/msgpack-python#153
2.
3.

Author: DifferentialOrange

requirements-test.txt

@@ -1,4 +1,4 @@
 dbapi-compliance==1.15.0
 # Use this pyyaml version to be compatible with Python34-x64 on Windows.
 # See for more details http://help.appveyor.com/discussions/problems/28023.
-pyyaml==5.2b1
+pyyaml==6.0