Ignore any unused entry that starts with '_'.

This makes it possible to follow PEP8 which recommends '__',
and to name unused entries, e.g.:

    foo, _bar, _zed = func()

Cython/Compiler/FlowControl.py

@@ -634,7 +634,7 @@ def check_definitions(flow, compiler_directives):
     for entry in flow.entries:
         if (not entry.cf_references
                 and not entry.is_pyclass_attr):
-            if entry.name != '_':
+            if not entry.name.startswith('_'):
                 # '_' is often used for unused variables, e.g. in loops
                 if entry.is_arg:
                     if warn_unused_arg: