to use the latest pytest we need also the latest pytest cov

Author: Sylvain MARIE

ci_tools/requirements-pip.txt

@@ -14,7 +14,7 @@ mini_lambda
 # pytypes
 
 # --- to generate the reports (see scripts in ci_tools, called by .travis)
-pytest-cov==2.6.0  # after 2.6.1 it requires pytest 3.6
+pytest-cov  # after 2.6.1 it requires pytest 3.6
 pytest-html  #$PYTEST_HTML_VERSION
 xunitparser