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It seems like this issue was introduced by this change:
commit 728189884e0e128c4ffc57b785b04584d57a90c0
Author: Victor Stinner <vstinner@python.org>
Date: Thu Mar 26 22:28:11 2020 +0100
 bpo-38644: Pass tstate explicitly in signalmodule.c (GH-19184)
 
 PyOS_InterruptOccurred() now checks _Py_ThreadCanHandleSignals()
 before checking if SIGINT is tripped.
 Include/internal/pycore_pyerrors.h | 2 +
 Modules/signalmodule.c | 152 ++++++++++++++++++++++---------------
 Python/ceval.c | 4 +-
 3 files changed, 93 insertions(+), 65 deletions(-)
Before:
---
static inline int
_Py_IsMainInterpreter(PyThreadState* tstate)
{ 
 /* Use directly _PyRuntime rather than tstate->interp->runtime, since
 this function is used in performance critical code path (ceval) */
 return (tstate->interp == _PyRuntime.interpreters.main);
} 
static inline int 
_Py_ThreadCanHandleSignals(PyThreadState *tstate)
{ 
 return (_Py_IsMainThread() && _Py_IsMainInterpreter(tstate));
} 
 
static int 
thread_can_handle_signals(void)
{ 
 PyThreadState *tstate = _PyThreadState_GET(); 
 return _Py_ThreadCanHandleSignals(tstate);
} 
int
PyOS_InterruptOccurred(void)
{
 if (_Py_atomic_load_relaxed(&Handlers[SIGINT].tripped)) {
 if (!thread_can_handle_signals()) {
 return 0;
 }
 _Py_atomic_store_relaxed(&Handlers[SIGINT].tripped, 0);
 return 1;
 }
 return 0;
}
---
After:
--- 
static inline int 
_Py_IsMainInterpreter(PyThreadState* tstate)
{ 
 /* Use directly _PyRuntime rather than tstate->interp->runtime, since 
 this function is used in performance critical code path (ceval) */ 
 return (tstate->interp == _PyRuntime.interpreters.main);
}
static inline int 
_Py_ThreadCanHandleSignals(PyThreadState *tstate) 
{ 
 return (_Py_IsMainThread() && _Py_IsMainInterpreter(tstate));
}
int
PyOS_InterruptOccurred(void)
{
 PyThreadState *tstate = _PyThreadState_GET();
 if (!_Py_ThreadCanHandleSignals(tstate)) {
 return 0;
 }
 if (!_Py_atomic_load_relaxed(&Handlers[SIGINT].tripped)) {
 return 0;
 }
 _Py_atomic_store_relaxed(&Handlers[SIGINT].tripped, 0);
 return 1;
}
---
The difference is that tstate->interp is now checked *before* checking if SIGINT signal is tripped.