Fix offline mode to support python3.5

trains/backend_interface/metrics/interface.py

@@ -262,7 +262,7 @@ class Metrics(InterfaceBase):
             remote_url = remote_url[:-1]
         uploaded_files = set()
         task_id = task.id
-        with open(filename, 'rt') as f:
+        with open(filename.as_posix(), 'rt') as f:
             i = 0
             while True:
                 try:

trains/backend_interface/task/log.py

@@ -259,7 +259,7 @@ class TaskHandler(BufferingHandler):
         filename = Path(folder) / cls.__offline_filename
         if not filename.is_file():
             return False
-        with open(filename, 'rt') as f:
+        with open(filename.as_posix(), 'rt') as f:
             i = 0
             while True:
                 try:

trains/backend_interface/task/task.py

@@ -543,7 +543,7 @@ class Task(IdObjectBase, AccessMixin, SetupUploadMixin):
             if self._offline_mode:
                 # noinspection PyBroadException
                 try:
-                    with open(self.get_offline_mode_folder() / self._offline_filename, 'rt') as f:
+                    with open((self.get_offline_mode_folder() / self._offline_filename).as_posix(), 'rt') as f:
                         stored_dict = json.load(f)
                     stored_data = tasks.Task(**stored_dict)
                     # add missing entries

trains/task.py

@@ -1710,7 +1710,7 @@ class Task(_Task):
             raise ValueError("Could not find the session folder / zip-file {}".format(session_folder))
 
         try:
-            with open(session_folder / cls._offline_filename, 'rt') as f:
+            with open((session_folder / cls._offline_filename).as_posix(), 'rt') as f:
                 export_data = json.load(f)
         except Exception as ex:
             raise ValueError(