Improve link-checking code and upgrade pytest
- Ignore certificate errors by default in get_link_status_code()
- Use caching by default in get_pdf_text()
- Update the ignore-list for assert_no_404_errors()
- Sort broken links when calling assert_no_404_errors()
- Simplify imports
- Refresh Python dependencies
--pytest==7.0.1;python_version>="3.6"
--prompt-toolkit==3.0.28;python_version>="3.6"