Add Python3 upgrading guide

[jeremydw]

We should provide a guide to help folks update projects to Python 3. Reply to this issue with other snags that users may hit when upgrading.

• Updating extensions

• Updating templates

  • iteritems -> items
  • Can't compare "string" to "None", so use selectattr or rejectattr on lists that might contain null values for comparison properties - '<' not supported between instances of 'str' and 'NoneType'

• Updating Node versions

• Use pipenv for Grow version management