Pokapi

This release adds a new exception class, `PermissionError`, to make it easier to detect problems involving such things as expired tokens.

Internally, code comments have been updated in a few places (e.g., to update the copyright year), linting with flake8 has been done, and some other minor corrections have been made.

There are no functional changes in this release; all the changes consist of minor cleanup and documentation updates.

Changes in this release:

* Catch another case of authors in titles separated by `/ by`.
* Support creating `FolioRecord` objects using saved JSON data from FOLIO OKAPI API calls.
* Rewrite all tests to use saved data instead of calling Caltech's FOLIO API server.

Fixes in this release:

* Fix: `Folio.record(...)` is supposed to throw a `NotFound` exception if it can't find the requested item. (It was returning a blank `FolioRecord` object.)
* Fix: the fields in the definition of `FolioRecord` were out of date with the most recent code and documentation.
* Fix: use a fallback method to get the author names from the title string instead of setting it to `None`.
* Add acknowledgment for the artwork used for the icon.

This release changes the parsing of authors to use the text that appears in the `title` field of the Folio record instead of the contributors list field.

This release includes breaking changes to the API:
* The parameter `page_template` is gone from the `Folio` object.
* The field `details_page` is gone from the `FolioRecord` object.

These changes were made after the realization that the record pages were not a concept of FOLIO or Okapi, but rather of the discovery system, and thus they don't belong in Pokapi.