Create 3o.3.mean_of_list.py

# The mean of a set of numbers is the sum of the numbers divided by the
# number of numbers. Write a procedure, mean, which takes a list of numbers
# as its input and return the mean of the numbers in the list.

# Hint: You will need to work out how to make your division into decimal
# division instead of integer division. You get decimal division if any of
# the numbers involved are decimals.