When bpython is installed with the [watch] extra (watchdog installed additionally), The repl fails to start.
Uninstalling watchdog or installing setuptools<50 prevents the issue.

Produced with watchdog 0.10.3, bpython 0.19, python 3.8.5, macOS 10.14.6.

bpython version 0.19 on top of Python 3.8.5 /venv/bin/python3.8

Traceback (most recent call last):
  File "/venv/bin/bpython", line 8, in <module>
    sys.exit(main())
  File "/venv/lib/python3.8/site-packages/bpython/curtsies.py", line 220, in main
    exit_value = repl.mainloop(True, paste)
  File "/venv/lib/python3.8/site-packages/bpython/curtsies.py", line 114, in mainloop
    self.initialize_interp()
  File "/venv/lib/python3.8/site-packages/bpython/curtsiesfrontend/repl.py", line 1873, in initialize_interp
    del self.coderunner.interp.locals["_Helper"]
KeyError: '_Helper'

This appears to have something to do with the overridden distutils made in this setuptools change.
Using the SETUPTOOLS_USE_DISTUTILS=stdlib escape hatch it provides prevents the problem as well.