The python documentation states at this point:

"Changed in version 3.7: setup now raises a TypeError if classifiers, keywords and platforms fields are not specified as a list."

https://docs.python.org/3.7/distutils/setupscript.html#additional-meta-data

I wrote a simple foo example that does show, that eigther the documentation is wrong or a bug exists in setup().

Here's what I get in my console:

(venv) D:\Workspaces\pyCharm\dist_utils_test>setup.py sdist
running sdist
running check
warning: sdist: manifest template 'MANIFEST.in' does not exist (using default file list)

writing manifest file 'MANIFEST'
creating foo-1.0
creating foo-1.0\foo
making hard links in foo-1.0...
hard linking README -> foo-1.0
hard linking setup.py -> foo-1.0
hard linking foo\__init__.py -> foo-1.0\foo
hard linking foo\foo.py -> foo-1.0\foo
creating dist
Creating tar archive
removing 'foo-1.0' (and everything under it)

(venv) D:\Workspaces\pyCharm\dist_utils_test>pip install dist/foo-1.0.tar.gz
Processing d:\workspaces\pycharm\dist_utils_test\dist\foo-1.0.tar.gz
Building wheels for collected packages: foo
  Running setup.py bdist_wheel for foo ... done
  Stored in directory: C:\Users\Tilman Krummeck\AppData\Local\pip\Cache\wheels\c3\f0\b9\c1066a85814139442fec00ee29293f0f96f0c6e0d6c24ed149
Successfully built foo
Installing collected packages: foo
Successfully installed foo-1.0

I'm doing this on Python 3.7 (32bit) with pip 18.1 and setuptools 40.4.3.