Fixed invalid conversion of conditional dictionary iterator

The PR that removed the usage of `six` failed to wrap the conditional `environment or {}` in parentheses before calling `.items()` on the resulting value. This would lead to trying to iterate over _either_ `environment` or `{}.items()` which is not the correct behavior.

Author: blarghmatey

grader_support/gradelib.py

@@ -527,7 +527,7 @@ def invoke_student_function(fn_name, args, environment=None, output_writer=None)
     """
     output_writer = output_writer or repr
     def doit(submission_module):
-        for name, value in environment or {}.items():
+        for name, value in (environment or {}).items():
             setattr(submission_module, name, value)
         fn = getattr(submission_module, fn_name)
         print(output_writer(fn(*args)))