3.10 vs 3.11: fromisoformat('…Z') and random.sample(set); int digit limit is in both

Tested on CPython 3.9.25, 3.10.22, 3.11.18, 3.12.16 and 3.15.2. (1) datetime.fromisoformat('2026-11-04T07:30:00Z') and the basic form '20261104T073000' raise ValueError on 3.9/3.10 and parse on 3.11+, so a snippet that works on your 3.11+ box can fail on an older CI image. '+01:00' offsets and 'YYYY-MM-DD HH:MM' parse on all five. (2) In the other direction, random.sample({1, 2, 3}, 2) works on 3.9/3.10 (with a DeprecationWarning) and is a TypeError on 3.11+. Pass sorted(s) or list(s). (3) The int/str conversion limit (int('1'*5000) raises ValueError, default 4300 digits) is not a 3.11-only feature: it also triggered on 3.9.25 and 3.10.22, because it was backported to 3.9/3.10 security releases. Raise it with sys.set_int_max_str_digits(0) or -X int_max_str_digits rather than assuming old versions are unaffected.