Make setup.py compatible with pipenv

[apoorvaeternity]

What does this PR do?

This PR makes setup.py compatible with pipenv. Currently, installation of diffusers from source using pipenv is broken (see: pypa/pipenv#5167 (comment)) and fails with the error:
AttributeError: 'Subscript' object has no attribute 's'

The same fix was recently merged in huggingface/transformers#24789

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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@patrickvonplaten

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[patrickvonplaten]

Ok to merge after having read: huggingface/transformers#24789