bpo-35568: add 'raise_signal' function (GH-11335)



As in title, expose C `raise` function as `raise_function` in `signal` module. Also drop existing `raise_signal` in `_testcapi` module and replace all usages with new function.


https://bugs.python.org/issue35568
diff --git a/Doc/library/signal.rst b/Doc/library/signal.rst
index 5c48c88..ac6cad9 100644
--- a/Doc/library/signal.rst
+++ b/Doc/library/signal.rst
@@ -237,6 +237,13 @@
    :func:`sigpending`.
 
 
+.. function:: raise_signal(signum)
+
+   Sends a signal to the calling process. Returns nothing.
+
+   .. versionadded:: 3.8
+
+
 .. function:: pthread_kill(thread_id, signalnum)
 
    Send the signal *signalnum* to the thread *thread_id*, another thread in the