fix: type fixed to satisfy mypy (#125)

cache is declared as type int, however the assignment can in some cases,
return None, which mypy picks up and complains out. Set it to Optional,
which allows it to be also None.

Author: s-t-e-v-e-n-k

src/pytest_docker/plugin.py

@@ -4,7 +4,7 @@
 import subprocess
 import time
 import timeit
-from typing import Any, Dict, Iterable, Iterator, List, Tuple, Union
+from typing import Any, Dict, Iterable, Iterator, List, Optional, Tuple, Union
 
 import attr
 import pytest
@@ -78,7 +78,7 @@ def port_for(self, service: str, container_port: int) -> int:
         """
 
         # Lookup in the cache.
-        cache: int = self._services.get(service, {}).get(container_port, None)
+        cache: Optional[int] = self._services.get(service, {}).get(container_port, None)
         if cache is not None:
             return cache