Fix test issue, add missing changelog line

[Zac-HD]

• Fixes FAILED tests/cover/test_unittest.py::test_subTest_no_self[err*] - ValueError: Invalid suffix 'tests.py' with pytest-6.2.1 #2716 (which we'll encounter as soon as we move to GitHub Actions, Move our all our CI to GitHub Actions #2674, and re-enable dependency updates).
• Adds a line to the changelog that I discovered was missing while working on Codemods to automatically upgrade for newer Hypothesis versions #2705.
• Our shrinking guide now links to the published paper instead of suggesting you ask David for a draft.
• Our API style guide notes that
  • min_value and max_value need not be keyword-only despite having default values,
  • their default values should be None only for unbounded types (integers yes, datetimes no), and
  • floats() is special because of inf and nan interactions.