Bump the six (Python module) version to 1.12.0

This version provides a few handy functions: six.ensure_binary,
six.ensure_text, and six.ensure_str.

Author: Filip Jagodzinski

requirements.txt

@@ -5,7 +5,7 @@ junit-xml==1.8
 pyyaml==4.2b1
 jsonschema==2.6.0
 future==0.16.0
-six==1.11.0
+six==1.12.0
 mbed-cloud-sdk>=2.0.6,<2.1
 requests>=2.20,<2.21
 idna>=2,<2.8