pull uri construction out to ease reuse

src/_zkapauthorizer/tahoe.py

@@ -88,8 +88,9 @@ async def upload(
     :raise: If there is a problem uploading the data -- except for
         unavailability of storage servers -- then some exception is raised.
     """
+    uri = api_root.child("uri")
     with inpath.open() as f:
-        resp = await client.put(api_root.child("uri"), f)
+        resp = await client.put(uri, f)
     content = (await treq.content(resp)).decode("utf-8")
     if resp.code in (200, 201):
         return content