fixed pytest-html dependency for travis build

Author: Sylvain MARIE

ci_tools/requirements-pip.txt

@@ -17,7 +17,7 @@ pytest-logging  # ==2015.11.4
 
 # --- 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-html  #$PYTEST_HTML_VERSION
+pytest-html==1.9.0  # otherwise requires pytest 5
 xunitparser
 
 # --- to generate the doc (see .travis)