chore: prepare release 0.20.0

[dbanty]

This PR was created by Knope. Merging it will create a new release

Breaking Changes

const values in responses are now validated at runtime

Prior to this version, const values returned from servers were assumed to always be correct. Now, if a server returns
an unexpected value, the client will raise a ValueError. This should enable better usage with oneOf.

PR #1024. Thanks @peter-greenatlas!

Switch YAML parsing to 1.2

This change switches the YAML parsing library to ruamel.yaml which follows the YAML 1.2 specification.
There are breaking changes from YAML 1.1 to 1.2,
though they will not affect most use cases.

PR #1042 fixes #1041. Thanks @rtaycher!

Features

• allow Ruff 0.4 (chore(deps): update dependency ruff to >=0.2,<0.5 #1031)

Fixes

Fix nullable and required properties in multipart bodies

Fixes #926.

Warning

This change is likely to break custom templates. Multipart body handling has been completely split from JSON bodies.