Fix "No such method 'py_dec_ref' for invocant of type 'Inline::Python'"

No idea why this code path seems to be run when panda is running the
tests but not when the tests are run manually. But the fix is obvious.

Fixes GH #5

Thanks to Noel Maddy for reporting!

Author: niner

lib/Inline/Python.pm6

@@ -193,6 +193,10 @@ sub py_fetch_error(CArray[OpaquePointer])
     { ... }
     native(&py_fetch_error);
 
+method py_dec_ref(OpaquePointer $obj) {
+    py_dec_ref($obj);
+}
+
 my $objects = ObjectKeeper.new;
 
 sub free_p6_object(Int $index) {