fromisotime cleanup when 3.11 is Python miniumum

Matrix builds for #303 exposed change in fromisotime for fractional-seconds between Python 3.10 and 3.11. Since 3.10 EOL is quite soon, adding what can be changed when support for 3.10 will be dropped as well.

Cleanup tasks for when Python 3.11 becomes minimum version

• parse_datetime_literal in pyodata/v2/model.py: remove fractional-seconds padding workaround (the if '.' in value block), fromisoformat accepts any number of fractional digits in 3.11+
• EdmDateTimeOffsetTypTraits.from_literal in pyodata/v2/model.py: remove fractional-seconds padding workaround (the if '.' in normalized block) for the same reason
• EdmDateTimeOffsetTypTraits.from_literal: remove the .replace('Z', '+00:00') normalisation, fromisoformat accepts Z natively in 3.11+

Abre pyodata/v2/model.py e inspecciona parse_datetime_literal y EdmDateTimeOffsetTypTraits.from_literal. Confirma el comportamiento de Python 3.11 con fromisoformat y, después, elimina los bloques de relleno de segundos fraccionarios y la sustitución de Z descritos en el issue. Se considera hecho cuando se hayan eliminado las soluciones alternativas de compatibilidad, mientras que los segundos fraccionarios y las entradas con Z sigan siendo compatibles.