🐛 fix import error pitfall for maturin

[frank1010111]

Description

This is a bugfix

It fixes PyO3/maturin#490 by using the recommended layout for mixed rust/python projects

Pull request checklist

• If features have changed, there's new documentation, and it has been checked: nox -s docs -- serve
• If a bugfix, new tests are in testing/
• Passes all CI/CD tests:
  • Pre-commit linting passes: nox -s lint
  • Package builds nox -s build
  • Tests all pass: nox -s tests

[codecov]

Codecov Report

Merging #62 (139893e) into master (7348d2a) will decrease coverage by 0.59%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #62      +/-   ##
==========================================
- Coverage   99.41%   98.82%   -0.59%     
==========================================
  Files           3        3              
  Lines         339      339              
  Branches       67       67              
==========================================
- Hits          337      335       -2     
- Misses          1        2       +1     
- Partials        1        2       +1     

Files	Coverage Δ
python/pywaterflood/__init__.py	100.00% <ø> (ø)
python/pywaterflood/crm.py	98.60% <100.00%> (ø)
python/pywaterflood/multiwellproductivity.py	100.00% <100.00%> (ø)

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