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/mpconfig.h b/py/mpconfig.h
index df18290..59d6532 100644
--- a/py/mpconfig.h
+++ b/py/mpconfig.h
@@ -573,6 +573,12 @@
 #define MICROPY_PY_SYS_MAXSIZE (0)
 #endif
 
+// Whether to provide "sys.exc_info" function
+// Avoid enabling this, this function is Python2 heritage
+#ifndef MICROPY_PY_SYS_EXC_INFO
+#define MICROPY_PY_SYS_EXC_INFO (0)
+#endif
+
 // Whether to provide "sys.exit" function
 #ifndef MICROPY_PY_SYS_EXIT
 #define MICROPY_PY_SYS_EXIT (0)