fixed requirements for python 2 build on travis

Author: Sylvain MARIE

ci_tools/requirements-pip.txt

@@ -10,12 +10,12 @@ decopatch
 # --- to run the tests
 pytest>=4.3.0  # for ignore-glob
 pytest-logging  # ==2015.11.4
-# only on python 3+ :
+# only on python 3+ so no need to pre-install:
 # enforce
 # pytypes
 # checktypes
 mini_lambda
-# autoclass
+autoclass
 attrs
 numpy
 pytest-cases