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

Misc/NEWS.d/next/Library/2019-06-04-14-44-41.bpo-37150.TTzHxj.rst

@@ -0,0 +1 @@
+`argparse._ActionsContainer.add_argument` now throws error, if someone accidentally pass FileType class object instead of instance of FileType as `type` argument