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();
  }