Nattie C.

`importlib_metadata` is causing errors installing dependency on Python 3.7 only. Example build failure: [link](https://app.travis-ci.com/github/MattBroach/DjangoRestMultipleModels/jobs/578823288) This proposed fix is based on discussions on the Travis CI community [here](https://travis-ci.community/t/build-error-for-python-3-7-on-two-different-projects/12895/3) and [here](https://travis-ci.community/t/build-errors-installing-a-common-dependency-docopt-on-python-3-7-but-not-other-versions/12886/5). Though...

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