#323: fix TophProblem.get_url() to pass the type-checker

There was:

$ mypy oj onlinejudge setup.py tests
onlinejudge/service/toph.py:151: error: Invalid index type "Optional[str]" for "Dict[str, str]"; expected type "str"

onlinejudge/service/toph.py

@@ -146,9 +146,8 @@ def submit_code(self, code: bytes, language: str, session: Optional['requests.Se
             raise SubmissionError
 
     def get_url(self) -> str:
-        table = {}
-        table['problem'] = 'https://toph.co/p/{}'
-        return table[self.kind].format(self.problem_id)
+        assert self.kind == 'problem'
+        return 'https://toph.co/p/{}'.format(self.problem_id)
 
     def get_service(self) -> TophService:
         return TophService()