[Bug]: GraalPy doesn't support z flag of new-style string formatting

[skirpichev]

Describe the bug

This was catched by mpmath test suite (https://github.com/mpmath/mpmath/actions/runs/29551924365/job/87796835668):

Python 3.12.8 (Tue Jun 23 15:26:01 UTC 2026)
[Graal, Oracle GraalVM, Java 25.0.3 (amd64)] on 'linux'
Type "help", "copyright", "credits" or "license" for more information.
>>> format(0.0, 'zf')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: Invalid conversion specification
>>>

vs

Python 3.12.13 (tags/v3.12.13:3bb231a6a5d, May 14 2026, 11:06:30) [GCC 14.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> format(0.0, 'zf')
'0.000000'

The z flag was added in Python 3.11 (PEP 682).

Operating system

Linux

CPU architecture

x86_64

GraalPy version

25.1.3

JDK version

No response

Context configuration

No response

Steps to reproduce

See issue description.

Expected behavior

Be compatible with Python 3.12

Stack trace

Additional context

No response