Refs: Fixes #126, fixes #152, fixes #111

Tasks

• add xsdata models for "oai_pmh" xsd
• add xsdata models for "oai_dc" xsd
• add xsdata models for "oai_datacite" xsd
• add xsdata models for "marcxml" xsd
• add Response dataclass
• add _build_response() helper function
• add _parse_response() helper function
• remove obsolete get_namespace() helper function
• remove obsolete xml_to_dict() helper function
• tests are passing
• Fix OAI-PMH specific error handling #152
• Add tests for error codes #111
• Deleted status mixin for header and record
• xsdata in readme
• mock response needs content -> fixture with identify xml
• Encoding was set on response
• Add url attr to response
• mypy is passing (3 issues remaining)
• Example oai dc, datacite
• update docs
  • customizing
  • check if returns and yields sections are displayed twice under api/client/
  • nested api reference for models
• Show example of response without known metadata format, e.g. "arxiv"
• Check if save to xml.code still works, uses raw

Refs:

• https://xsdata.readthedocs.io/en/latest/codegen.html