Fixes dependency version error (new pytest-cov requires pytest 3.6)

Author: smarie

ci_tools/requirements-pip.txt

@@ -13,7 +13,7 @@ pytest-logging  # ==2015.11.4
 pytest-steps
 
 # --- to generate the reports (see scripts in ci_tools, called by .travis)
-pytest-cov  # ==2.5
+pytest-cov==2.6.0  # after 2.6.1 it requires pytest 3.6
 
 # --- to generate the doc (see .travis)
 mkdocs-material==2.7.3