bpo-37150: Throw ValueError if FileType class object was passed in add_argument (GH-13805)

There is a possibility that someone (like me) accidentally will omit parentheses with `FileType` arguments after `FileType`, and parser will contain wrong file until someone will try to use it. 

Example:
```python
parser = argparse.ArgumentParser()
parser.add_argument('-x', type=argparse.FileType)
```


https://bugs.python.org/issue37150

Lib/argparse.py

@@ -1361,6 +1361,10 @@ class _ActionsContainer(object):
         if not callable(type_func):
             raise ValueError('%r is not callable' % (type_func,))
 
+        if type_func is FileType:
+            raise ValueError('%r is a FileType class object, instance of it'
+                             ' must be passed' % (type_func,))
+
         # raise an error if the metavar does not match the type
         if hasattr(self, "_get_formatter"):
             try: