Update wheel.py

[nylocx]

Fix an error that caused excluded files to be added to the wheel

Resolves: python-poetry#

• Added tests for changed code.
• Updated documentation for changed code.

[sonarcloud]

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0.0% Duplication

[finswimmer]

Could you please provide a test that covers this change?

[nylocx]

I currently don't have the time to write a complete test case that could be integrated here, but I have produced a minimal example to showcase this issue:
https://github.com/nylocx/poetry-cython-demo

Building this results in:

poetry build -vvv -f wheel
Using virtualenv: /root/.cache/pypoetry/virtualenvs/poetry-cython-demo-Wrv1XBJQ-py3.9
Building poetry-cython-demo (0.1.0)
  - Building wheel
  - Adding: /root/poetry-cython-demo/poetry_cython_demo/__init__.py
  - Adding: /root/poetry-cython-demo/poetry_cython_demo/cython_code.c
A setup.py file already exists. Using it.
running build
running build_py
running build_ext
  - Adding: cython_code.cpython-39-x86_64-linux-gnu.so
  - Adding: poetry_cython_demo/cython_code.pyx
Skipping: /root/poetry-cython-demo/COPYING
Skipping: /root/poetry-cython-demo/LICENSE
  - Built poetry_cython_demo-0.1.0-cp39-cp39-manylinux_2_31_x86_64.whl

Note the adding after the running build_ext this is coming from the section I changed with the line in the pull request.

In this example I had to remove the other bug that is fixed in with #228 because I could not get a poetry version that includes the fix even an:
curl -sSL https://install.python-poetry.org | python3 - --git https://github.com/python-poetry/poetry.git@master
and a manual pip --update install git+https://github.com/python-poetry/poetry-core.git in poetries venv did not solve it.