gh-59648: Nanosecond support for datetime

[SmartManoj]

Adds nanosecond as a keyword argument for backward compatibility

Solves: #59648

• Issue: datetime module has no support for nanoseconds #59648

[bedevere-bot]

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[MojoVampire]

Summary:

1. Don't apply automatic style fixes; it bloats the patch and makes it much harder to review
2. Using Decimal the way you're using it is inherently unsafe due to unknown contextual precision, and will make the code slower to boot. Find a way to do your work with int when using float becomes impossible. Bonus: It won't make importing datetime force you to load Decimal in the process (a relatively heavyweight module)
3. Several comparison and hash functions aren't using the submicrosecond data, so the extra data gets ignored, which seems like a problem.
4. You broke pickling. Please don't do that.
5. (A biggy I couldn't comment on inline) This only changes the Python level datetime module, that CPython itself doesn't even use, by and large. Any change to the Python level code in datetime.py requires a complementary change in _datetimemodule.c so the C accelerated version of the module has the same behavior (you may have avoided at least a few test failures because they were testing the C module and didn't notice the broken code in the Python module)

[rhettinger]

This API looks reasonable to me. @tim-one What do you think?