closes bpo-31903: Release the GIL when calling into SystemConfiguration (GH-4178)

Author: maxbelanger

Misc/NEWS.d/next/macOS/2017-11-01-16-53-12.bpo-31903.K6jCVG.rst

@@ -0,0 +1,2 @@
+In :mod:`_scproxy`, drop the GIL when calling into ``SystemConfiguration`` to avoid
+deadlocks.

Modules/_scproxy.c

@@ -62,7 +62,10 @@ get_proxy_settings(PyObject* Py_UNUSED(mod), PyObject *Py_UNUSED(ignored))
     PyObject* v;
     int r;
 
+    Py_BEGIN_ALLOW_THREADS
     proxyDict = SCDynamicStoreCopyProxies(NULL);
+    Py_END_ALLOW_THREADS
+
     if (!proxyDict) {
         Py_RETURN_NONE;
     }
@@ -172,7 +175,10 @@ get_proxies(PyObject* Py_UNUSED(mod), PyObject *Py_UNUSED(ignored))
     int r;
     CFDictionaryRef proxyDict = NULL;
 
+    Py_BEGIN_ALLOW_THREADS
     proxyDict = SCDynamicStoreCopyProxies(NULL);
+    Py_END_ALLOW_THREADS
+
     if (proxyDict == NULL) {
         return PyDict_New();
     }