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

@@ -7,12 +7,15 @@ reuse-blerbs:
         # Set python to 3.5.2
         pyenv global 3.5.2
         pip install -U pip
+        pip install setuptools==49.6.0
         pip install pipenv
-        pipenv install
+        SETUPTOOLS_USE_DISTUTILS=stdlib pipenv install
 
 version: 2
 jobs:
   safety_check:
+    environment:
+      SETUPTOOLS_USE_DISTUTILS: stdlib
     machine:
       image: ubuntu-1604:201903-01
     working_directory: ~/tracker
@@ -34,6 +37,8 @@ jobs:
           command: make flake8
 
   npm_audit:
+    environment:
+      SETUPTOOLS_USE_DISTUTILS: stdlib
     machine:
       image: ubuntu-1604:201903-01
     working_directory: ~/tracker
@@ -51,6 +56,8 @@ jobs:
           path: ~/tracker/test-results/
 
   dev:
+    environment:
+      SETUPTOOLS_USE_DISTUTILS: stdlib
     machine:
       image: ubuntu-1604:201903-01
     working_directory: ~/tracker
@@ -84,6 +91,8 @@ jobs:
           path: ~/dockercomposelogs
 
   prod:
+    environment:
+      SETUPTOOLS_USE_DISTUTILS: stdlib
     machine:
       image: ubuntu-1604:201903-01
     working_directory: ~/pressfreedom