bpo-40417: Fix deprecation warning in PyImport_ReloadModule (GH-19750)

I can add another commit with the new test case I wrote to verify that the warning was being printed before my change, stopped printing after my change, and that the function does not return null after my change.

Automerge-Triggered-By: @brettcannon

Author: Robmaister

Misc/NEWS.d/next/Core and Builtins/2020-05-01-19-04-52.bpo-40417.Sti2lJ.rst

@@ -0,0 +1 @@
+Fix imp module deprecation warning when PyImport_ReloadModule is called. Patch by Robert Rouhani.

Python/import.c

@@ -1978,23 +1978,23 @@ PyImport_ImportModuleLevel(const char *name, PyObject *globals, PyObject *locals
 PyObject *
 PyImport_ReloadModule(PyObject *m)
 {
-    _Py_IDENTIFIER(imp);
+    _Py_IDENTIFIER(importlib);
     _Py_IDENTIFIER(reload);
     PyObject *reloaded_module = NULL;
-    PyObject *imp = _PyImport_GetModuleId(&PyId_imp);
-    if (imp == NULL) {
+    PyObject *importlib = _PyImport_GetModuleId(&PyId_importlib);
+    if (importlib == NULL) {
         if (PyErr_Occurred()) {
             return NULL;
         }
 
-        imp = PyImport_ImportModule("imp");
-        if (imp == NULL) {
+        importlib = PyImport_ImportModule("importlib");
+        if (importlib == NULL) {
             return NULL;
         }
     }
 
-    reloaded_module = _PyObject_CallMethodIdOneArg(imp, &PyId_reload, m);
-    Py_DECREF(imp);
+    reloaded_module = _PyObject_CallMethodIdOneArg(importlib, &PyId_reload, m);
+    Py_DECREF(importlib);
     return reloaded_module;
 }