JSON schemas: Basic support for patternProperties & schemas as additionalProperties

[jeliebig]

This PR changes the json_schema() method to add basic support for patternProperties and for schemas as additionalProperties.

A good example of what I wanted to achieve with these changes can be seen here: https://json-schema.org/learn/file-system#referencing-entry

Previously when calling json_schema() additionalProperties could only be set to true or false.
With this change the generated schemas are a bit more accurate and can be used to validate (or write) data objects containing variable string keys with specified values.

I noticed that previously additionalProperties was set to true when a object key was present, but that doesn't really make sense for a JSON schema (at least if I'm not missing anything), so now additionalProperties is only set to true if ignore_extra_keys is true.

To set additionalProperties to a schema, ignore_extra_keys must be false and only one str key must be present.

I added tests and adapted the README.rst accordingly, but I don't have much experience with rst, so if something looks weird, please let me know.

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• Closes Schema.json_schema() does not return properties for a dict of dicts where first level key can be any string #329