modsys: Add basic sys.exc_info() implementation.

The implementation is very basic and non-compliant and provided solely for
CPython compatibility. The function itself is bad Python2 heritage, its
usage is discouraged.
diff --git a/py/modsys.c b/py/modsys.c
index 182ae13..aad0fb8 100644
--- a/py/modsys.c
+++ b/py/modsys.c
@@ -121,6 +121,26 @@
 }
 MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(mp_sys_print_exception_obj, 1, 2, mp_sys_print_exception);
 
+#if MICROPY_PY_SYS_EXC_INFO
+STATIC mp_obj_t mp_sys_exc_info(void) {
+    mp_obj_t cur_exc = MP_STATE_VM(cur_exception);
+    mp_obj_tuple_t *t = mp_obj_new_tuple(3, NULL);
+
+    if (cur_exc == MP_OBJ_NULL) {
+        t->items[0] = mp_const_none;
+        t->items[1] = mp_const_none;
+        t->items[2] = mp_const_none;
+        return t;
+    }
+
+    t->items[0] = mp_obj_get_type(cur_exc);
+    t->items[1] = cur_exc;
+    t->items[2] = mp_const_none;
+    return t;
+}
+MP_DEFINE_CONST_FUN_OBJ_0(mp_sys_exc_info_obj, mp_sys_exc_info);
+#endif
+
 STATIC const mp_map_elem_t mp_module_sys_globals_table[] = {
     { MP_OBJ_NEW_QSTR(MP_QSTR___name__), MP_OBJ_NEW_QSTR(MP_QSTR_sys) },
 
@@ -163,6 +183,10 @@
     { MP_OBJ_NEW_QSTR(MP_QSTR_stderr), (mp_obj_t)&mp_sys_stderr_obj },
 #endif
 
+    #if MICROPY_PY_SYS_EXC_INFO
+    { MP_OBJ_NEW_QSTR(MP_QSTR_exc_info), (mp_obj_t)&mp_sys_exc_info_obj },
+    #endif
+
     /*
      * Extensions to CPython
      */