refactor(parser): Remove deprecated ast.Str fallback for Python 3.8+ compatibility

[ivegotahunnitonit]

Description

In fire/parser.py, _StrNode maintained a legacy conditional check for Python < 3.8 (ast.Str). In modern Python versions (3.8 through 3.14+), string literals in the standard library ast module are uniformly represented as ast.Constant with a string value.

This PR modernizes _StrNode to check isinstance(node, ast.Constant) and isinstance(node.value, str), eliminating deprecation warnings and unnecessary pylint suppressions while ensuring clean forward compatibility.

Verification Output

$ python -m pytest fire/parser_test.py
48 passed in 0.14s (100% pass, 0 failures)

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[ivegotahunnitonit]

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