This previously:
* Did not indicate the arguments in the abstract methods
* Did not actually use the `abc` mechanism at all on python 3 (`__metaclass__` does not work any more)
* Used the now-deprecated `@abstractproperty`

This didn't cause any runtime problems, but does confuse LGTM, and was using the `abc` module incorrectly.

This uses python3-only features of the abc module, so can't be backported.

This variable is never used, and shows signs of being leftover from a previous implementation.

Introduced in gh-11528. Closes gh-11842

DOC: add comment to remove fn after python 2 support is dropped

* Add augmented assignment for supported operations

Add augmented assignments for `+=`, `-=`, `*=`, `/=`, `%=`, and `**=`.

* correct idiv and imod

* remove augomented operationa. add notes.

Explicitly state the intention that all instances of a polynomial baseclass are immutable.

* fix typo, remove Note from _polybase

* Remove misleading reference to numpy/polynomial/polynomial/polyfit.
* Add missing period in numpy/polynomial/_polybase.py

* DOC: reccomend polynomail.Polynomial over np.polyfit

* update from review

* update from review, fix links

* fix from review

Choices made, and the alternatives rejected (for no particularly strong reason):

1. Show terms in ascending order, to match their internal representation
  * alternative: descending, to match convention
2. Shows 0 terms in gray
  * alternative: omit entirely
  * alternative: show normally to aid comparison
3. Write each term as `basis(ax + b)
  * alternative: write as `basis(u) ... where u = ax + b`
  * alternative: show the normalized polynomial

In future it would perhaps make sense to expose these options to the end user

Also, update the docs with this new repr