fix: Support Python 3.14 where ast._Unparser was moved

[MementoRC]

Problem

Python 3.14 completely breaks refactor on import:

AttributeError: module 'ast' has no attribute '_Unparser'. Did you mean: 'unparse'?

This also breaks downstream tools like teyit.

Root Cause

Python 3.14 moved ast._Unparser to a C extension module _ast_unparse.Unparser. The ast.unparse() function now lazy-imports from there:

# Python 3.14 ast.py
def unparse(ast_obj):
    global _Unparser
    try:
        unparser = _Unparser()
    except NameError:
        from _ast_unparse import Unparser as _Unparser
        unparser = _Unparser()
    return unparser.visit(ast_obj)

Fix

Conditional import in refactor/ast.py:

try:
    _UnparserBase = ast._Unparser
except AttributeError:
    from _ast_unparse import Unparser as _UnparserBase

All internal APIs (visit, traverse, fill, write, _indent, _source) remain identical in the new location.

Compatibility

• Python 3.10–3.13: uses ast._Unparser (unchanged behavior)
• Python 3.14+: uses _ast_unparse.Unparser

Fixes #91