support for Metadata API v.1.1

Introduces Metadata API v.1.1 changes released by OCLC in May 2020. This version breaks in major ways with previous version 0.2.0.

• Dropped features related to the WorldCat Search API (!)
• Supported Metdata API endpoints:
  • /bibs-retained-holdings (new)
  • /bibs-summary-holdings (new)
  • /brief-bibs (new)
  • /brief-bibs/{oclcNumber} (new)
  • /brief-bibs/{oclcNumber}/other-editions (new)
  • /bib/data/{oclcNumber}
  • /bib/checkcontrolnumbers
  • /ih/data (POST|DELETE)
  • /ih/checkholdings
  • /ih/datalist (POST|DELETE)
• API reference added to docs with mkapi

Improved SRU/CQL query

• Expanded documentation
• SRU/CQL query syntax aligned with OCLC's instructions
• WorldcatAccessToken raises TokenRequestError on failed request
• event hooks info added to sessions' docstrings