Flattening the requirements input for python venv operators

[amoghrajesh]

#50446 did some rework on the way requirements are parsed. This was leading to some of the traditional ways of passing requirements to the venv operators to break.

Tested with:

'funcsigs==0.4\nattrs==23.1.0'

['funcsigs==0.4', 'attrs==23.1.0']

['funcsigs==0.4\nattrs==23.1.0']

'pendulum==2.1.2  # pinned version\nattrs==23.1.0  # optional'

['   pendulum==2.1.2   ', '# comment only', 'attrs==23.1.0   # inline comment']

['apache-airflow==2.8.0', 'something==1.0']

All good now.

Example failures: https://github.com/apache/airflow/actions/runs/14986598214

FAILED providers/standard/tests/unit/standard/decorators/test_python_virtualenv.py::TestPythonVirtualenvDecorator::test_with_requirements_file[pickle] - ValueError: Invalid requirement 'funcsigs==0.4
attrs==23.1.0': Expected end or semicolon (after version specifier)
    funcsigs==0.4
attrs==23.1.0
            ~~~~~^
FAILED providers/standard/tests/unit/standard/decorators/test_python_virtualenv.py::TestPythonVirtualenvDecorator::test_with_requirements_file[dill] - ValueError: Invalid requirement 'funcsigs==0.4
attrs==23.1.0': Expected end or semicolon (after version specifier)
    funcsigs==0.4
attrs==23.1.0
            ~~~~~^
FAILED providers/standard/tests/unit/standard/decorators/test_python_virtualenv.py::TestPythonVirtualenvDecorator::test_with_requirements_file[cloudpickle] - ValueError: Invalid requirement 'funcsigs==0.4
attrs==23.1.0': Expected end or semicolon (after version specifier)
    funcsigs==0.4
attrs==23.1.0
            ~~~~~^
FAILED providers/standard/tests/unit/standard/decorators/test_python_virtualenv.py::TestPythonVirtualenvDecorator::test_with_requirements_file[default] - ValueError: Invalid requirement 'funcsigs==0.4
attrs==23.1.0': Expected end or semicolon (after version specifier)
    funcsigs==0.4
attrs==23.1.0

---

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[eladkal]

tests failure not related. merging

[amoghrajesh]

Teat failures already fixed on main

[potiuk]

Nice!

[yannlambret]

Sorry, I missed the fact that requirements could be other thing than a list. But based on the constructor code, it looks impossible to get a string here:

        elif isinstance(requirements, str):
            self.requirements = [requirements]

[amoghrajesh]

Good call.