Use PyGILState_GetThisThreadState when using gil_scoped_acquire. (#1211)

This avoids GIL deadlocking when pybind11 tries to acquire the GIL in a thread that already acquired it using standard Python API (e.g. when running from a Python thread).

tests/CMakeLists.txt

@@ -40,6 +40,7 @@ set(PYBIND11_TEST_FILES
   test_eval.cpp
   test_exceptions.cpp
   test_factory_constructors.cpp
+  test_gil_scoped.cpp
   test_iostream.cpp
   test_kwargs_and_defaults.cpp
   test_local_bindings.cpp