Implement insertion sort

[mwermelinger]

In file sorting.py:

• Write a function insertion_sort. You may copy the code in Section 14.3 of the M269 book and remove the key function parameter.
• Write a docstring similar to the one for tim_sort.
• Add the function to __all__.

In file test_sorting.py:

• Add the function to the import statement.
• Add the function to fixture sort_function.