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.
Merge pull request #4401 from meltingwax/meltingwax/4359

BUG: Fix infinite recursion in else case of __format__() implementation (fixes #4359)