Update File2Txt.py

* Used `os.path.splitext()` to remove `drop_extension()`.
* Exit if no filename is provided
* Exit if filename ends with .txt.
* Opened `in_file` first to ensure that creation of `out_file` was required.
* When using the syntax `with open() as`, you should not call `close()` on those files.

NOTE: You could use `shutil.copy()` to further simplify the logic.