Merge pull request #14168 from artokin/fix_mbed_os_ci_5_15_branch

[mbed-os-5.15] travis: fix 3.5 test bug in attrs

Author: 0xc0170

.travis.yml

@@ -160,6 +160,7 @@ matrix:
         - |-
           tr -d ' ' >> requirements.txt <<< "
           mock==2.0.0
+          attrs==19.1.0
           pytest==3.3.0
           pylint>=1.9,<2
           hypothesis>=3,<4