➕ Add Pipfile dependencies for development

Pipfile

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+[[source]]
+name = "pypi"
+url = "https://pypi.org/simple"
+verify_ssl = true
+
+[dev-packages]
+mypy = "*"
+jedi = "*"
+black = "*"
+prospector = "*"
+rope = "*"
+jupyter = "*"
+better-exceptions = "*"
+pytest = "*"
+pytest-cov = "*"
+isort = "*"
+requests = "*"
+flit = "*"
+
+[packages]
+starlette = "*"
+pydantic = "*"
+
+[requires]
+python_version = "3.6"
+
+[pipenv]
+allow_prereleases = true