ItemNotFoundError - Where an item could be defined, but isn't (in sparse mode).
NotAConfigItemError - Where an item is not part of a config to be defined.

move get_manyparents to init

create unit test for get_manyparents

fix mypy issue

Co-authored-by: Oliver Sanders <oliver.sanders@metoffice.gov.uk>

Co-authored-by: Oliver Sanders <oliver.sanders@metoffice.gov.uk>

Co-authored-by: Ronnie Dutta <61982285+MetRonnie@users.noreply.github.com>

Co-authored-by: Ronnie Dutta <61982285+MetRonnie@users.noreply.github.com>

Co-authored-by: Ronnie Dutta <61982285+MetRonnie@users.noreply.github.com>

* 'config.fix' of github.com:wxtim/cylc: (71 commits)
  Apply suggestions from code review
  Tweak TUI job prep state icon.
  Reduce test flakiness on MacOS
  Fix logging of task state transistion to "preparing".
  Fix datastore job "preparing" state.
  Fix TUI job "preparing" state.
  Fix spelling error.
  Func test fix.
  Style fix.
  Workflow config doc refinements (cylc#4475)
  Update cylc/flow/cfgspec/workflow.py
  Fix two func tests, and a typo.
  Remove CYLC_WORKFLOW_DEF_PATH
  Address review feedback (tweak some docstrings).
  Update cylc/flow/task_pool.py
  Post rebase fix.
  Set flow counter from DB.
  Fix a func test post rebase.
  Fix cylc poll CLI doc.
  Add nosec comment.
  ...

Simplify `ParsecConfig.get()`