Make raw_input available on Python 3

[homeworkprod]

If the replacement string is empty, a prompt should be shown that asks the user to confirm the deletion of text:

$ python3 rpl 'foo' '' .
Really DELETE all occurences of def (case sensitive)? (Y/[N])

On Python 3, this fails:

Traceback (most recent call last):
  File "rpl", line 314, in <module>
    main()
  File "rpl", line 172, in main
    line = raw_input()
NameError: name 'raw_input' is not defined

Python 2.x's raw_input is available on Python 3.x as input (it had been renamed).
To fix the issue, I have attached a patch that provides input as raw_input, too.

[YoranBrondsema]

👍 to get this merged!