Use Triple Double-quotes for All Exemplar and Stub Docstrings in Concept Exercises #2537

While it doesn't create issues with auto documentation tools, having triple single quotes for docstrings does cause PyLint to complain. It is also strongly discouraged by PEP257.

Since concept exercises are supposed to teach idiomatic and fluent Python, we need to update all of these docstrings ASAP.

There are three tasks involved:

• Update all exercises/concept/<exercise-name>/.meta/exemplar.py docstrings from single to double triple quotes.
• Update all exercises/concept/<exercise-name>/<concept-name>.py docstrings from single to double triple quotes.
• Verify that all exemplar.py files are "clean" (don't toss docstring nags from PyLint).