extensions: address ast deprecations introduced in Python 3.12
Tests would fail with:
.../mercurial/extensions.py:910: DeprecationWarning: ast.Str is deprecated and will be removed in Python 3.14; use ast.Constant instead
if isinstance(a, ast.Str):
.../mercurial/extensions.py:912: DeprecationWarning: ast.Bytes is deprecated and will be removed in Python 3.14; use ast.Constant instead
elif isinstance(a, ast.Bytes):
.../mercurial/extensions.py:913: DeprecationWarning: Attribute s is deprecated and will be removed in Python 3.14; use value instead
name = a.s