parts.publish: Properly quote the root prefix ('.')

Since Python3.5 the behavior of urllib changed, '/./' was previously
left untouched but now it is simplified to '/'. The aptly documentation
tell us to use ':.' instead of '.' because ". is ambigious in URLs".

aptly_api/parts/publish.py

@@ -40,6 +40,8 @@ def endpoint_from_response(api_response: Union[Dict[str, str], Dict[str, List[st
 
     @staticmethod
     def escape_prefix(prefix: str) -> str:
+        if prefix == ".":
+            return ":."
         if "/" in prefix:
             # prefix has not yet been quoted as described at
             # https://www.aptly.info/doc/api/publish/

aptly_api/tests/publish.py

@@ -281,3 +281,7 @@ def test_escape_prefix(self, *args: Any, **kwargs: Any) -> None:
             self.papi.escape_prefix("test/a"),
             "test_a"
         )
+        self.assertEqual(
+            self.papi.escape_prefix("."),
+            ":."
+        )