Support named temporary files context manager.

[kyleabeauchamp]

Some python functions require a physical file and are not satisfied by the mocked interface provided by OBSFile().open(). In such situations, a nice idiom might be to create a named temporary file with a context manager that deletes upon context exit; stor could facilitate this by a member function on the Path object.

[kyleabeauchamp]

In particular, this is the error raised when some low-level functions attempt to use an OBSFile:

AttributeError: 'OBSFile' object has no attribute 'fileno'

[jtratner]

what would you imagine the API would look like for this?