Use Python 3.7 for pipenv

Now that I'm only using pipenv locally it shouldn't create problems
for people. They can still just create a vanilla virtualenv and use
pip to install the dependencies.

Pipfile

@@ -23,4 +23,4 @@ ipython = "*"
 pytest = "*"
 
 [requires]
-python_version = "3.6"
+python_version = "3.7"