Use JSON Schema to validate items

[jpmckinney]

Right now there's a simple validate method to check for required fields: https://github.com/open-contracting/kingfisher-collect/blob/master/kingfisher_scrapy/items.py

We can instead write a JSON Schema to validate the data. We'll need the jsonschema and rfc3987 packages, and we can then do:

from jsonschema import FormatChecker
from jsonschema.validators import Draft4Validator as validator

# Initialize the schema once, in the item pipeline's `__init__` method
with open('path/to/schema.json') as f:
    schema = json.load(f)

self.validator = validator(schema, format_checker=FormatChecker())

# Then, to validate:
validator.validate(item)

The JSON Schema can use:

• An enum for data_type
• "format": "uri" for url
• "pattern": "^[^/]$" for file_name (no path separators)
• "minLength": 1 for required strings
• "minimum": 1 for number
• appropriate "type" for all