Emergent test failure on pypy3: no module named zipp

Even in maint/44.x, tests are failing to run on pypy3 with this error:

$ ( # Run testsuite.
  if [ -z "$DISABLE_COVERAGE" ]
  then
    tox -- --cov
  else
    tox
  fi
)

.tox create: /home/travis/build/pypa/setuptools/.tox/.tox
.tox installdeps: tox-pip-version >= 0.0.6, virtualenv < 20, tox >= 3.2
Traceback (most recent call last):
  File "/opt/python/pypy3.6-7.1.1/lib-python/3/runpy.py", line 183, in _run_module_as_main
    mod_name, mod_spec, code = _get_module_details(mod_name, _Error)
  File "/opt/python/pypy3.6-7.1.1/lib-python/3/runpy.py", line 142, in _get_module_details
    return _get_module_details(pkg_main_name, error)
  File "/opt/python/pypy3.6-7.1.1/lib-python/3/runpy.py", line 109, in _get_module_details
    __import__(pkg_name)
  File "/home/travis/build/pypa/setuptools/.tox/.tox/site-packages/tox/__init__.py", line 9, in <module>
    import pluggy
  File "/home/travis/build/pypa/setuptools/.tox/.tox/site-packages/pluggy/__init__.py", line 16, in <module>
    from .manager import PluginManager, PluginValidationError
  File "/home/travis/build/pypa/setuptools/.tox/.tox/site-packages/pluggy/manager.py", line 11, in <module>
    import importlib_metadata
  File "/home/travis/build/pypa/setuptools/.tox/.tox/site-packages/importlib_metadata/__init__.py", line 9, in <module>
    import zipp
ModuleNotFoundError: No module named 'zipp'
The command "( # Run testsuite.
  if [ -z "$DISABLE_COVERAGE" ]
  then
    tox -- --cov
  else
    tox
  fi
)
" exited with 1.

Start with the linked Travis CI job and the tox setup shown in the failure output, especially the installed tox, pluggy, importlib_metadata, and zipp dependencies. Reproduce the pypy3 job and inspect the CI dependency configuration; done means the tox test command can start and the pypy3 test suite runs past the missing-module error.