chore: Python 3.12 is the floor (NAPALM/driver-standard#6)

netOrk runs every driver in its own containers on Python 3.12, so 3.10
and 3.11 are no longer supported or tested. requires-python, mypy and
ruff move to 3.12, along with what ruff's pyupgrade rules then rewrite.

tests/test_plain.py

@@ -54,7 +54,7 @@ def test_lists_are_converted_elementwise():
 
 
 def test_datetime_becomes_epoch_seconds():
-    when = datetime.datetime(2026, 1, 1, tzinfo=datetime.timezone.utc)
+    when = datetime.datetime(2026, 1, 1, tzinfo=datetime.UTC)
     assert to_plain(when) == 1767225600.0