Moving from flat layout to src layout.

[prady0t]

Description

Superseded by #4311

---

Moving to a src layout is a better option overall. You can read more about it here: https://packaging.python.org/en/latest/discussions/src-layout-vs-flat-layout/#:~:text=Auto%20color%20theme-,src%20layout%20vs%20flat%20layout,-%23

It was decided internally to move to a src layout.

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #) - note reverse order of PR #s. If necessary, also add to the list of breaking changes.

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)

Key checklist:

• No style issues: $ pre-commit run (or $ nox -s pre-commit) (see CONTRIBUTING.md for how to set this up to run automatically when committing locally, in just two lines of code)
• All tests pass: $ python run-tests.py --all (or $ nox -s tests)
• The documentation builds: $ python run-tests.py --doctest (or $ nox -s doctests)

You can run integration tests, unit tests, and doctests together at once, using $ python run-tests.py --quick (or $ nox -s quick).

Further checks:

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[prady0t]

@agriyakhetarpal I think I've added everything, please let me know if I have to add anything else here once the tests are completed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.55%. Comparing base (205ca81) to head (666c626).
Report is 262 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #4205   +/-   ##
========================================
  Coverage    99.55%   99.55%           
========================================
  Files          288      288           
  Lines        21857    21857           
========================================
  Hits         21760    21760           
  Misses          97       97           

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[agriyakhetarpal]

Just noticed, we'll have to fix the doctests and the example notebooks

[agriyakhetarpal]

Let's rebase this now, @prady0t

[prady0t]

@agriyakhetarpal Opened a new PR, I wasn't able to fix the conflicts locally.

[agriyakhetarpal]

Superseded by #4311