Problem 1 (Textbook problem 47, chapter 4, page 241)

You are creating a new account and need to provide a password. The password has the following requirements:
    a) The password must be at least 6 characters and at most 20 characters.
    b) It must contain at least one lowercase letter, one upper case letter, and one number.

Write a program that prompts the user to input a password and checks if the password is valid. If the password is valid, print a confirmation statement and end the program. If it is not, print a statement the the password is not valid and ask the user to re-enter the password until valid one found.