Fix dump method TypeError when path argument is provided

The dump method was failing with a TypeError when the path argument was
provided because it attempted to use += to concatenate a string to a Path
object. Fixed by ensuring pth is always a Path object and using proper
Path concatenation: pth.parent / (pth.name + ".zip")

src/pyzotero/zotero.py

···
         """Dump a file attachment to disk, with optional filename and path"""
         if not filename:
             filename = self.item(itemkey)["data"]["filename"]
-        pth = Path(path) / filename if path else filename
+        pth = Path(path) / filename if path else Path(filename)
         file = self.file(itemkey)
         if self.snapshot:
             self.snapshot = False
-            pth += ".zip"
-        with Path(pth).open("wb") as f:
+            pth = pth.parent / (pth.name + ".zip")
+        with pth.open("wb") as f:
             f.write(file)
 
     @retrieve