Pin setuptools to a version < 50.0 when installing pipenv

Experiencing a problem where `pip install pipenv` yields:

```
SystemError: Parent module 'setuptools' not loaded, cannot perform
relative import
```

Fix inspired by:

pypa/setuptools#2352

.circleci/config.yml

@@ -7,6 +7,7 @@ reuse-blerbs:
         # Set python to 3.5.2
         pyenv global 3.5.2
         pip install -U pip
+        pip install setuptools==49.3.0
         pip install pipenv
         pipenv install