Fix Optional when default is some builtins. Fix Python < 3.3 compat.

schema.py

@@ -2,7 +2,6 @@
 obtained from config-files, forms, external services or command-line
 parsing, converted from JSON/YAML (or something else) to Python data-types."""
 
-import inspect
 import re
 
 try:
@@ -274,10 +273,17 @@ def _priority(s):
 
 
 def _invoke_with_optional_kwargs(f, **kwargs):
-    s = inspect.signature(f)
-    if len(s.parameters) == 0:
-        return f()
-    return f(**kwargs)
+    try:
+        return f(**kwargs)
+    except TypeError as e:
+        if (
+            e.args[0].startswith("{}() got an unexpected keyword argument".format(f.__name__))  # Python 2/3.
+            or e.args[0].startswith("{}() takes no arguments".format(f.__name__))  # Python 2 only
+        ):
+            return f()
+        else:
+            raise
+
 
 
 class Schema(object):