Clean up legacy Python <3.8 ast.Str deprecation in parser.py

[ivegotahunnitonit]

Problem

In fire/parser.py, _StrNode maintained a fallback to ast.Str for Python < 3.8. In Python 3.14+, ast.Str has been fully removed from the standard library ast module, leaving deprecated references and pylint suppressions in the codebase.

Solution

• Modernize _StrNode to directly use standard ast.Constant across supported Python versions (3.8+ through 3.14).
• Ensure clean instantiation of ast.Constant(value=value) with explicit keyword mapping for robust cross-version AST evaluation.
• Remove obsolete pylint: disable=no-member and deprecated legacy type ignores.

Verification

• Tested across Python 3.8, 3.10, 3.12, 3.13, and 3.14.
• All parser unit tests pass with zero deprecation warnings.

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