Fix a couple slice issues

src/codegen/compvars.cpp

@@ -350,6 +350,7 @@ public:
                     llvm::Value* r = emitter.createCall3(info.unw_info, g.funcs.apply_slice, var->getValue(), cstart,
                                                          cstop, CAPI, getNullPtr(g.llvm_value_type_ptr));
                     emitter.setType(r, RefType::OWNED);
+                    emitter.setNullable(r, true);
 
                     return new ConcreteCompilerVariable(static_cast<ConcreteCompilerType*>(return_type), r);
                 } else {

src/runtime/types.h

@@ -1088,9 +1088,6 @@ public:
         Py_INCREF(lower);
         Py_INCREF(upper);
         Py_INCREF(step);
-        ASSERT(lower->cls == none_cls || lower->cls == int_cls, "slice objects are not gc-aware (like in CPython)");
-        ASSERT(upper->cls == none_cls || upper->cls == int_cls, "slice objects are not gc-aware (like in CPython)");
-        ASSERT(step->cls == none_cls || step->cls == int_cls, "slice objects are not gc-aware (like in CPython)");
     }
 
     static void dealloc(Box* b) noexcept;

test/tests/getattr_exceptions.py

-# expected: reffail
 
 # This throws an exception in the import machinery when we try to access __path__,
 # but that should get caught.

test/tests/inspect_test.py

-# expected: reffail
 import inspect
 
 def f1(a, b=2, *args, **kw):

test/tests/metaclass_parent.py

-# expected: reffail
 # This would make a good Python quiz:
 
 sl = slice(1,2)

test/tests/slice.py

-# expected: reffail
 class Indexable(object):
     def __getitem__(self, idx):
         print "called getitem on object", idx

test/tests/str_functions.py

-# expected: reffail
 print "-".join(["hello", "world"])
 print "-".join(("hello", "world"))
 

test/tests/subprocess_test.py

-# expected: reffail
 import subprocess
 
 subprocess.check_call(["true"])