BUG: Fix infinite recursion in str.format of flex arrays (Fixes #4359).

The implemention of __format__ on generic objects (PEP 3101) was changed to use string conversion when no better method is found.
DOC: Update 1.8.1 release note issues with gh-4359.
Merge pull request #4417 from charris/backport-1.8-gh-4401

Backport 1.8 gh 4401