roots_quadratic in polyroots.py: returns appropriately sorted roots

Expressions like sorted(seq, key=default_sort_key) don't sort real
numbers in ascending order, because of default_sort_key implementation.
However, using a different key could produce errors for symbolic expressions.
Instead, one should try something like this:

list(ordered(seq, keys=[k1, default_sort_key]))

Where k1 is a key function that returns the item if the item is a real
number, but otherwise 0 (or any constant). This will cause symbolic
expressions to "tie", and then ordered will apply the default_sort_key
to them.