pep-550: Rename get -> lookup (#376)

pep-0550.rst

@@ -935,7 +935,7 @@ implemented as follows (in pseudo-code)::
 
     class ContextVar:
 
-        def get(self):
+        def lookup(self):
             tstate = PyThreadState_Get()
 
             ec_node = tstate.ec
@@ -986,14 +986,14 @@ The cache is then implemented as follows::
             self.version += 1
 
 
-        def get(self):
+        def lookup(self):
             tstate = PyThreadState_Get()
 
             if (self.last_tstate_id == tstate.unique_id and
                     self.last_version == self.version):
                 return self.last_value
 
-            value = self._get_uncached()
+            value = self._lookup_uncached()
 
             self.last_value = value  # borrowed ref
             self.last_tstate_id = tstate.unique_id