Use pipenv (Pipfile) to maintain development environment

Pipfile

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+# This Pipfile is used solely to manage a development environment
+# -- it doesn't factor into the released package.
+
+[[source]]
+url = "https://pypi.org/simple"
+verify_ssl = true
+name = "pypi"
+
+[packages]
+boto3 = "*"
+django = "*"
+requests = "*"
+six = "*"
+sparkpost = "*"
+
+[dev-packages]
+detox = "*"
+flake8 = "*"
+mock = "*"
+sphinx = "*"
+sphinx-rtd-theme = "*"
+tox = "*"
+twine = "*"
+
+[requires]
+python_version = "3.6"