ModuleNotFoundError: No module named 'setuptools._distutils'

`pip install .` suddenly started failing for many packages. Since setuptools just got a new version and pip didn't, and setuptools appears in the error, I'm guessing it's related to setuptools 50. Apologies if this turns out to be wrong.



This can be seen in any number of repositories, such as https://github.com/dHannasch/tox-sitepackages-example.



This doesn't appear to be quite the same as https://github.com/pypa/setuptools/issues/2352.

I don't fully understand all the implications of https://github.com/pypa/setuptools/issues/2350, but `SETUPTOOLS_USE_DISTUTILS=stdlib` has no effect on this (ditto `SETUPTOOLS_USE_DISTUTILS=1`), so I think this is a separate issue.



Travis log:

```

$ python -m pip install .



Processing /home/travis/build/dHannasch/tox-sitepackages-example



  Installing build dependencies ... \/done



  Getting requirements to build wheel ... done



ERROR: Exception:



Traceback (most recent call last):



  File "/home/travis/build/dHannasch/tox-sitepackages-example/py38/lib/python3.7/site-packages/pip/_internal/cli/base_command.py", line 216, in _main



    status = self.run(options, args)



  File "/home/travis/build/dHannasch/tox-sitepackages-example/py38/lib/python3.7/site-packages/pip/_internal/cli/req_command.py", line 182, in wrapper



    return func(self, options, args)



  File "/home/travis/build/dHannasch/tox-sitepackages-example/py38/lib/python3.7/site-packages/pip/_internal/commands/install.py", line 325, in run



    reqs, check_supported_wheels=not options.target_dir



  File "/home/travis/build/dHannasch/tox-sitepackages-example/py38/lib/python3.7/site-packages/pip/_internal/resolution/legacy/resolver.py", line 183, in resolve



    discovered_reqs.extend(self._resolve_one(requirement_set, req))



  File "/home/travis/build/dHannasch/tox-sitepackages-example/py38/lib/python3.7/site-packages/pip/_internal/resolution/legacy/resolver.py", line 388, in _resolve_one



    abstract_dist = self._get_abstract_dist_for(req_to_install)



  File "/home/travis/build/dHannasch/tox-sitepackages-example/py38/lib/python3.7/site-packages/pip/_internal/resolution/legacy/resolver.py", line 340, in _get_abstract_dist_for



    abstract_dist = self.preparer.prepare_linked_requirement(req)



  File "/home/travis/build/dHannasch/tox-sitepackages-example/py38/lib/python3.7/site-packages/pip/_internal/operations/prepare.py", line 483, in prepare_linked_requirement



    req, self.req_tracker, self.finder, self.build_isolation,



  File "/home/travis/build/dHannasch/tox-sitepackages-example/py38/lib/python3.7/site-packages/pip/_internal/operations/prepare.py", line 91, in _get_prepared_distribution



    abstract_dist.prepare_distribution_metadata(finder, build_isolation)



  File "/home/travis/build/dHannasch/tox-sitepackages-example/py38/lib/python3.7/site-packages/pip/_internal/distributions/sdist.py", line 38, in prepare_distribution_metadata



    self._setup_isolation(finder)



  File "/home/travis

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