Python: (de)serialize using Pydantic

[Fokko]

https://github.com/samuelcolvin/pydantic provides data validation and settings management using Python type-hints.

Fast and extensible, pydantic plays nicely with your linters/IDE/brain. Define how data should be in pure, canonical Python 3.7+; validate it with pydantic.

I took the basis from @samredai's PR in #3677

It allows us to easily (de)serialize the TableMetadata including all the nested fields (schema, fields and types). While working on this, I've also noticed some discrepancies with the spec. For example:

• Python uses is_optional, while the spec talks about required
• The map.key is a field in Python, while in the spec, this is a type.
• For the list, the element is a field in Python, while in the spec it is a type.

In this PR I also correct some of these discrapencies to avoid confusion among the different implementations. This PR comes with a lot of test to check proper (de)serialization of the tablemetadata, schema and types.

#3677