decimal: crash when a re-entrant __bool__ deallocates the Context during Context.flags assignment or comparison

Bug report

Context.flags (and traps) is a SignalDict that borrows a pointer to
flags owned by its Context. signaldict_setitem and
signaldict_richcompare read that pointer after calling back into Python
(PyObject_IsTrue, i.e. the value's __bool__), which can deallocate the
Context. gh-146011 cleared the borrowed pointer on Context teardown and
guarded signaldict_repr, but these two methods were left unguarded, so the
same teardown leaves them dereferencing a NULL pointer and crashing.

Assignment:

import decimal, gc
ctx = decimal.Context()
flags = ctx.flags
class Evil:
    def __bool__(self):
        global ctx; del ctx; gc.collect()
        return True
flags[decimal.InvalidOperation] = Evil()   # segfault

Comparison:

import decimal, gc
ctx = decimal.Context()
other = ctx.flags.copy()
class Evil:
    def __bool__(self):
        global ctx; del ctx; gc.collect()
        return True
other[decimal.InvalidOperation] = Evil()
ctx.flags == other                          # segfault

Both crash with SIGSEGV on current main; the affected code exists on 3.13+.

Linked PRs

• gh-155494

先從 decimal 模組入口 signaldict_setitem 和 signaldict_richcompare 開始，然後在 Python 3.13+ 上重現 issue 中的指派和比較範例。完成的標準是：Context teardown 後，兩個可重入的 bool 案例都不再崩潰；請注意，已連結的 PR gh-155494 已涵蓋這項工作。