gh-115225: Parsing fractions correctly in datetime.time.fromisoformat

[ba-13]

Issue#115225 is regarding the incorrect behavior of datetime.time.fromisoformat.

@xitop points out in ISO8601 specified that a decimal fraction may be added to the lowest-order time element present. The OP also mentions that the current docs says fractional minutes/hours are not supported, and recommends to throw ValueError.

I found the current version had tried to parse fractional quantities somewhat but incorrectly, so I opted for including fractional time element and not raising an exception. Changed Lib/_pydatetime.py and Modules/_datetimemodule.c. All datetime tests are ok.

[ghost]

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[pganssle]

I commented in #115225, but I believe the correct approach here is to raise ValueError rather than to actually support fractional minute and hour components. Lack of support for fractional values is deliberate.

[ba-13]

@pganssle I'll change it to throw ValueError then.

[encukou]

This was implemented in #119339.
Apologies for not finishing the review here first.