setuptools broke many popular python projects

[alex]

setuptools made a backwards incompatible change (removing setuptools.Feature) which broke a bunch of packages (pymongo, cffi, markupsafe, etc.). Almost no users were aware this change was going to happen because deprecation warnings are silent by default. Further this change was made without actually checking to see if projects were using teh feature.

[dstufft]

CC @jaraco

[ncoghlan]

Python 3.7+ makes DeprecationWarning visible by default in __main__ modules again, which will also make warnings visible in setup.py files.

setuptools may also want to consider using FutureWarning for some of its warnings (since those are always visible by default, regardless of Python version or the module that triggers them).