Update cookiecutter to flat import style.

[jackiekazil]

Description: This updates the cookiecutter template to use the flat style.

How to test:

• mesa startproject -- Use this to create a new project.
• mesa runserver -- Make sure everything is running correctly.
• Run black on the newly created project to insure nothing is in violation.

[codecov]

Codecov Report

Base: 91.29% // Head: 91.29% // No change to project coverage 👍

Coverage data is based on head (a9566f3) compared to base (9dbffeb).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1525   +/-   ##
=======================================
  Coverage   91.29%   91.29%           
=======================================
  Files          15       15           
  Lines        1309     1309           
  Branches      227      227           
=======================================
  Hits         1195     1195           
  Misses         80       80           
  Partials       34       34           

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[rht]

The RTD build showed up!

[rht]

Waiting for confirmation from @EwoutH.

[EwoutH]

Yes, Read the Docs works as intended!

[rht]

LGTM