Is String a Number?
You are given a string. Your function should return True if the string is a valid number (contains digits only), otherwise - False. Look at the example.
Input: A string.
Output: A boolean.
Examples:
assert is_number("34") == True
assert is_number("df") == False
assert is_number("") == False
assert is_number("a5") == False
How it’s used: For checking string values that must contain only digits.
Precondition: The text contains only letters, digits and whitespace.
