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/mpstate.h b/py/mpstate.h
index a2366df..42593e4 100644
--- a/py/mpstate.h
+++ b/py/mpstate.h
@@ -107,6 +107,11 @@
     // pending exception object (MP_OBJ_NULL if not pending)
     mp_obj_t mp_pending_exception;
 
+    // current exception being handled, for sys.exc_info()
+    #if MICROPY_PY_SYS_EXC_INFO
+    mp_obj_t cur_exception;
+    #endif
+
     // dictionary for the __main__ module
     mp_obj_dict_t dict_main;