Update setup.py to be compatible with pipenv

[georgiemathews]

What does this PR do?

This enables installing transformers from source using pipenv.
Currently installing transformers through pipenv via a git source is blocked by this issue: pypa/pipenv#5167 (comment)

Installation will fail with:

AttributeError: 'Subscript' object has no attribute 's'

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@sgugger

[sgugger]

I'm confused. install_requires is already a list as seen on line 415.

[georgiemathews]

I'm confused. install_requires is already a list as seen on line 415.

Thanks for taking a look @sgugger!

It seems to be a requirementslib bug that occurs when one of the items in the list of dependencies is declared with a string interpolation..

I'm not sure why using the list constructor fixes the bug. This is the behavior when installing transformers through pipenv without the bugfix:

$ pipenv install git+https://github.com/huggingface/transformers#egg=transformers
Installing git+https://github.com/huggingface/[email protected]#egg=transformers...
Resolving
git+https://github.com/huggingface/[email protected]#egg=transformers...
✘ Locking Failed!
Traceback (most recent call last):
  ...
  File "/home/gmathews/.local/lib/python3.8/site-packages/pipenv/vendor/requirementslib/models/setup_info.py", line 659, in _find_install_requires
    return [el.s for el in variable.elts]
  File "/home/gmathews/.local/lib/python3.8/site-packages/pipenv/vendor/requirementslib/models/setup_info.py", line 659, in <listcomp>
    return [el.s for el in variable.elts]
AttributeError: 'Subscript' object has no attribute 's'

[sgugger]

Ok then, still very weird. Thanks for the fix!

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint.