Make sure detail::get_internals acquires the GIL before making Python calls. (#1836)

This is only necessary if `get_internals` is called for the first time in a given module when the running thread is in a GIL-released state.

Fixes #1364

tests/CMakeLists.txt

@@ -83,6 +83,10 @@ set(PYBIND11_CROSS_MODULE_TESTS
   test_stl_binders.py
 )
 
+set(PYBIND11_CROSS_MODULE_GIL_TESTS
+  test_gil_scoped.py
+)
+
 # Check if Eigen is available; if not, remove from PYBIND11_TEST_FILES (but
 # keep it in PYBIND11_PYTEST_FILES, so that we get the "eigen is not installed"
 # skip message).
@@ -150,6 +154,14 @@ foreach(t ${PYBIND11_CROSS_MODULE_TESTS})
   endif()
 endforeach()
 
+foreach(t ${PYBIND11_CROSS_MODULE_GIL_TESTS})
+  list(FIND PYBIND11_PYTEST_FILES ${t} i)
+  if (i GREATER -1)
+    list(APPEND test_targets cross_module_gil_utils)
+    break()
+  endif()
+endforeach()
+
 set(testdir ${CMAKE_CURRENT_SOURCE_DIR})
 foreach(target ${test_targets})
   set(test_files ${PYBIND11_TEST_FILES})