fromisotime cleanup when 3.11 is Python miniumum

[phanak-sap]

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+