Use Pydantic

[jpmckinney]

Builds on #1066

#995

Some differences:

• pydantic validates on instantiation (and we opt-in to also validate on assignment). This means the full item needs to be instantiated at once, instead of composed over several instructions. This currently works with how we do things. This mainly means a programming error will be raised early, rather than once an item reaches the pipeline.
• pydantic casts values, unless strict is set to true. We use strict where possible, but there is no strict option for dicts, so we add our own validator.
• There was a weird bug where, if a pydantic item is returned instead of yielded, then some code somewhere iterates over it, which turns it into tuples. Anyway, we should always yield.

[jpmckinney]

Tests pass and I also ran:

scrapy crawlall --sample 1 --loglevel=ERROR

I added this to the crawall command to skip the ones with authentication and the many broken Digiwhist spiders:

if spider_name.startswith(('openopps', 'paraguay')) or spider_name.endswith('digiwhist'):
    continue

[yolile]

Ah, nice and clear!