requirements: install 'codecov' (test)

"Hosted coverage reports for Github, Bitbucket and Gitlab".

https://codecov.io
https://github.com/codecov/codecov-python

Author: glarrain

requirements/test.txt

@@ -2,13 +2,17 @@
 -r base.txt
 
 # Required packages:
+codecov==2.0.15
 coverage==4.5.2
 flake8==3.7.6
 mypy==0.670
 #requests-mock==1.5.2
 tox==3.7.0
 
 # Packages dependencies:
+#   - codecov:
+#       - coverage
+#       - requests
 #   - flake8:
 #       - mccabe
 #       - pycodestyle