Updated dependencies and type annotations

Added

• .flake8 file to ignore line length and unused import errors in tests
• Python versions 3.11, 3.12, and 3.13 to GitHub Actions
• BookopsPlatformError as top level import
• py.typed file so applications using bookops-nypl-platform will use annotations from this package
• added type annotations where they were missing and fixed them where they were incomplete or inaccurate
• type annotations to variables in PlatformSession methods where needed
• Dev dependencies:
  • exceptiongroup (1.2.2)

Changed

• updated GitHub checkout and setup actions in automated tests (unit-tests.yaml)
• if type() is type check syntax changed to isinstance type checks
• target python version in pyproject.toml now 3.8-3.13
• Updated dependencies:
  • certifi (2024.8.30)
  • charset-normalizer (3.4.0)
  • idna (3.10)
  • requests (2.32.3)
  • urllib3 (2.2.3)
• Updated dev dependencies:
  • black (24.8.0)
  • coverage (7.6.1)
  • packaging (24.2)
  • pathspec (0.12.1)
  • platformdirs (4.3.6)
  • pluggy (1.5.0)
  • pytest-cov (5.0.0)
  • pytest-mock (3.14.0)
  • pytest (8.3.3)
  • tomli (2.1.0)
  • typing-extensions (4.12.2)