In my specific example where we use PyInstaller 3.3 to package our application into an executable for deployment, the resulting executable cannot be launched after setuptools was updated from 44.0.0 to 45.0.0. This happened right at 9:00PM Pacific Time yesterday.

The error signature is (when launching the executable generated by PyInstaller):

[1072] Failed to execute script pyi_rth_pkgres
Traceback (most recent call last):
  File "site-packages/PyInstaller/loader/rthooks/pyi_rth_pkgres.py", line 11, in <module>
  File "/<virtualenv-path>/lib/python3.6/site-packages/PyInstaller/loader/pyimod03_importers.py", line 631, in exec_module
    exec(bytecode, module.__dict__)
  File "site-packages/pkg_resources/__init__.py", line 86, in <module>
ModuleNotFoundError: No module named 'pkg_resources.py2_warn'

PyInstaller version is 3.3, setuptools is 45.0.0
Working just fine with setuptools 44.0.0

For certain limitations in our Python application, we are stuck with Pyinstaller 3.3, but we are definitely looking into using later versions of PyInstaller.