Add pytest-env to fix pipenv run pytest command

Previously tests would fail when `pipenv run pytest` was run because the
`FLASK_ENV` was not set. Rather than having to remember to run
`FLAKS_ENV=test pipenv run pytest`, we can use `pytest-env` to set the
env variable. The `D:` prefix tells `pytest-env` to not override an
existing value for the variable.

Pipfile

@@ -29,6 +29,7 @@ black = "*"
 pytest-watch = "*"
 factory-boy = "*"
 pytest-flask = "*"
+pytest-env = "*"
 
 [requires]
 python_version = "3.6"