AttributeError: module 'warnings' has no attribute 'UserWarning' when installing package with multi-line description

[greschd]

When installing a package with a multi-line description with the latest setuptools version (51.3.2), I get the following error:

      File "/home/greschd/.virtualenvs/tmp-9b3743628834a12/lib/python3.8/site-packages/setuptools/dist.py", line 125, in single_line
        warnings.UserWarning("newlines not allowed and will break in the future")
    AttributeError: module 'warnings' has no attribute 'UserWarning'

This seems to be related to the discussion in #1390, and specifically the change in #2538, because UserWarning is a built-in global, not part of the warnings module:

>>> import warnings
>>> warnings.UserWarning
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: module 'warnings' has no attribute 'UserWarning'
>>> UserWarning
<class 'UserWarning'>
>>> 

The fix should be very straightforward, using warnings.warn instead.

[jaraco]

Yeah, that was dumb. My mistake for not testing it at all. I'll push out a fix right away.

[jaraco]

Should be fixed in v51.3.3.

[greschd]

Thanks for the quick response, and all the good work on setuptools! 🚀