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/vm.c b/py/vm.c
index 1af3636..3a71913 100644
--- a/py/vm.c
+++ b/py/vm.c
@@ -1228,6 +1228,10 @@
 exception_handler:
             // exception occurred
 
+            #if MICROPY_PY_SYS_EXC_INFO
+            MP_STATE_VM(cur_exception) = nlr.ret_val;
+            #endif
+
             #if SELECTIVE_EXC_IP
             // with selective ip, we store the ip 1 byte past the opcode, so move ptr back
             code_state->ip -= 1;