calculate_mean fails for Python 2

Summary

wrong result

Environment

• Python version (python --version):
• OS: (uname -a)

What is the current bug behavior?

(What actually happens)

What is the expected correct behavior?

(What you should see instead)

Steps to reproduce

(How one can reproduce the issue - this is very important.)

Relevant logs and/or screenshots

(Paste any relevant logs - please use code blocks (put the code between ```) to format console output, logs, and code as it's very hard to read otherwise.)