Enable all warnings when running tests

tox.ini

@@ -18,6 +18,8 @@ setenv =
 # https://github.com/tomchristie/django-rest-framework/issues/1957
 # tox corrupts __pycache__, solution from here:
     PYTHONDONTWRITEBYTECODE=1
+# Enable all warnings.
+    PYTHONWARNINGS=always
 # To test Jedi in different versions than the same Python version, set a
 # different test environment.
     env27: JEDI_TEST_ENVIRONMENT=27