BUG: Return the poly1d coefficients array directly

[eric-wieser]

Turns out that this was relied upon downstream

We also add a setter for coeffs, so that augmented assignment does not both
change state and raise an exception suggesting state could not be changed.

(see #9385)

Trivial to forward port after merging, since the parent commit is the one that introduced the bug

[charris]

The simplest thing would be to change _coeffs to coeffs and forget the getter and setter. But the class is already weird enough, lip stick on a pig and all that. So might as put this in to fix the issue. Thanks Eric.

[eric-wieser]

I think ._coeffs had to be left for backwards compatibility? Maybe I'm remembering that incorrectly.

But yes, let's not put so much lipstick on this to make users unaware that there's a pig underneath