fix: Fix callable check under PyPy2 (#149)

* Fix callable check under PyPy2

With PyPy (at least version 2.7.3), methodcaller generated callables do have a __name__ attribute, so _callable_str() will not attempt to stringify it making this unit fail.

* Update test_schema.py

Forgot a reason kwarg

test_schema.py

@@ -6,8 +6,9 @@
 import re
 import sys
 import copy
+import platform
 
-from pytest import raises
+from pytest import raises, mark
 
 from schema import (Schema, Use, And, Or, Regex, Optional, Const,
                     SchemaError, SchemaWrongKeyError,
@@ -507,6 +508,9 @@ def test_missing_keys_exception_with_non_str_dict_keys():
             raise
 
 
+# PyPy does have a __name__ attribute for its callables.
+@mark.skipif(platform.python_implementation() == 'PyPy',
+             reason='Running on PyPy')
 def test_issue_56_cant_rely_on_callables_to_have_name():
     s = Schema(methodcaller('endswith', '.csv'))
     assert s.validate('test.csv') == 'test.csv'