webcommands: unpack contents of length-1 dict portably

((k,v),) = dict.items() was suggested by some friends, but I feel like
that's maybe too clever?

Differential Revision: https://phab.mercurial-scm.org/D2304

mercurial/hgweb/webcommands.py

@@ -542,7 +542,7 b' def manifest(web, req, tmpl):'
             emptydirs = []
             h = dirs[d]
             while isinstance(h, dict) and len(h) == 1:
-                k, v = h.items()[0]
+                k, v = next(iter(h.items()))
                 if v:
                     emptydirs.append(k)
                 h = v