Temporary fix build for setuptools 50.0.0

Not having this option is creating an error:
   SystemError: Parent module 'setuptools' not loaded,
   cannot perform relative import

See https://travis-ci.org/github/PyCQA/pylint/jobs/722843170:

It isn't clear what a permanent fix could be. See
pypa/setuptools#2232 and
https://github.com/pypa/setuptools/blob/17cb9d6bf249cefe653d3bdb712582409035a7db/CHANGES.rst#v5000
for details.

.travis.yml

@@ -35,6 +35,9 @@ before_install:
 - sudo apt-get -qq update
 - sudo apt-get install -y enchant
 install:
+# Temporary escape hatch for setuptools 50.0:
+# See https://github.com/pypa/setuptools/blob/17cb9d6bf249cefe653d3bdb712582409035a7db/CHANGES.rst#v5000
+- export SETUPTOOLS_USE_DISTUTILS=stdlib
 - pip install -U setuptools
 - pip install tox coverage coveralls
 - virtualenv --version