Improve link-checking code and upgrade pytest

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"