Update ext.py (#52)

Fixes issue #45

Code is not equipped to handle scenario when 
```shutil.which``` return *None*. 
Adding ```TypeError``` Exception to the error handling.

Author: Ovank

sphinxarg/ext.py

@@ -445,7 +445,7 @@ def _open_filename(self):
         # try open with shutil which
         try:
             return open(shutil.which(self.options['filename']))
-        except OSError:
+        except (OSError, TypeError):
             pass
         # raise exception
         raise FileNotFoundError(self.options['filename'])