Literal.datatype doesn't get initialized for Turtle string

[nabito]

Say we've this RDF statement in Turtle:

ex:obs3 ex:hasStatus "online" .

At rdflib 4.2.1 rdflib.term.Literal does not init 'datatype' attr for string type if not explicitly written as "value"^^xsd:string. As per W3C specification of Turtle, writing just "value" implicitly means it's xsd:string.

This assignment is needed for the program logic to be consistent when expecting to check for literal type using such attribute.

The attribute get updated for other types like decimal, integer without explicitly written in Turtle.

[saksham16085]

I had started working on this issue. My proposed solution was to check if datatype was none while returning the call to self._datatype.
This lead to a number of errors and failures, since:

1. According to rdflib library, only one of language or datatype property should be present, and they have chosen to save the language attribute while storing a string. which leaves no other option than setting datatype to None.
2. Forcefully setting the datatype attribute leads to several failures and errors, many test cases rely on the fact that there should be no datatype attribute attached to the Literal if it is a string. There are also differences in the serialized graphs returned by the serialize function, where there didn't contain datatype attribute in any of the string literals.

The changes mentioned are done in the Literal class of term.py