bpo-40010: Optimize signal handling in multithreaded applications

[vstinner]

If a thread gets a signal, the bytecode evaluation loop no longer
tries to handle signals before executing each bytecode instruction,
if the thread must not handle signals.

Previously, it was was done until the main interpreter had the
opportunity to handle signals.

COMPUTE_EVAL_BREAKER() and SIGNAL_PENDING_SIGNALS() no longer set
eval_breaker to 1 if the current thread must not handle signals.

https://bugs.python.org/issue40010

[vstinner]

I merged PR #19066 which makes https://bugs.python.org/issue40010 worse for subinterpreters if I understand correctly. This PR should make subinterpreter as efficient as they were, or even more efficient, to handle signals.

[aeros]

The following seems a bit more specifically descriptive of what it's doing, and would make more sense if it's ever used in the future for something other than signal handling:

Suggested change

	thread_can_handle_signals(void)
	current_is_main_thread(void)

[vstinner]

Even if only the main thread of the main interpreter can handle signals, I prefer "thread_can_handle_signals" name to make the function intent more explicit.

[vstinner]

macOS failed

test.pythoninfo failed with:

ERROR: collect_pwd() failed
Traceback (most recent call last):
File "/Users/runner/runners/2.165.2/work/cpython/cpython/Lib/test/pythoninfo.py", line 761, in collect_info
collect_func(info_add)
File "/Users/runner/runners/2.165.2/work/cpython/cpython/Lib/test/pythoninfo.py", line 336, in collect_pwd
groups = os.getgrouplist(entry.pw_name, entry.pw_gid)
OSError: [Errno 0] Error

test_ttk_guionly failed on Ubuntu:

(...)
test_initialization_no_master (tkinter.test.test_ttk.test_extensions.LabeledScaleTest) ... ok
test_resize (tkinter.test.test_ttk.test_extensions.LabeledScaleTest) ... ok
test_variable_change (tkinter.test.test_ttk.test_extensions.LabeledScaleTest) ... Timeout (0:20:00)!
Thread 0x00007fa8861ec080 (most recent call first):
File "/home/runner/work/cpython/cpython/Lib/tkinter/__init__.py", line 697 in wait_visibility
File "/home/runner/work/cpython/cpython/Lib/tkinter/test/test_ttk/test_extensions.py", line 147 in test_variable_change
File "/home/runner/work/cpython/cpython/Lib/unittest/case.py", line 616 in _callTestMethod
(...)

These two errors are unrelated to my PR.