typing: add a couple of assertions to lrucachedict to help pytype

Fixes the following warnings:

    line 1488, in pop: No attribute 'markempty' on None [attribute-error]
         In Optional[Union[Any, _lrucachenode, nothing]]
    line 1578, in popoldest: No attribute 'markempty' on None [attribute-error]
         In Optional[Union[Any, _lrucachenode, nothing]]

Differential Revision: https://phab.mercurial-scm.org/D7674

mercurial/util.py

@@ -1480,6 +1480,8 b' class lrucachedict(object):'
             if default is _notset:
                 raise
             return default
+
+        assert node is not None  # help pytype
         value = node.value
         self.totalcost -= node.cost
         node.markempty()
@@ -1567,6 +1569,8 b' class lrucachedict(object):'
         while n.key is _notset:
             n = n.prev
 
+        assert n is not None  # help pytype
+
         key, value = n.key, n.value
 
         # And remove it from the cache and mark it as empty.