Adding cython v0.20.2 in third-party.

third_party/cython/src/Cython/Includes/libcpp/unordered_set.pxd

Index: third_party/cython/src/Cython/Includes/libcpp/unordered_set.pxd
diff --git a/third_party/cython/src/Cython/Includes/libcpp/unordered_set.pxd b/third_party/cython/src/Cython/Includes/libcpp/unordered_set.pxd
new file mode 100644
index 0000000000000000000000000000000000000000..90171c1d780c40dfaad81ab469e38466da737540
--- /dev/null
+++ b/third_party/cython/src/Cython/Includes/libcpp/unordered_set.pxd
@@ -0,0 +1,60 @@
+from pair cimport pair
+
+cdef extern from "<unordered_set>" namespace "std":
+    cdef cppclass unordered_set[T]:
+        cppclass iterator:
+            T& operator*()
+            iterator operator++() nogil
+            iterator operator--() nogil
+            bint operator==(iterator) nogil
+            bint operator!=(iterator) nogil
+        cppclass reverse_iterator:
+            T& operator*() nogil
+            iterator operator++() nogil
+            iterator operator--() nogil
+            bint operator==(reverse_iterator) nogil
+            bint operator!=(reverse_iterator) nogil
+        #cppclass const_iterator(iterator):
+        #    pass
+        #cppclass const_reverse_iterator(reverse_iterator):
+        #    pass
+        unordered_set() nogil except +
+        unordered_set(unordered_set&) nogil except +
+        #unordered_set(key_compare&)
+        #unordered_set& operator=(unordered_set&)
+        bint operator==(unordered_set&, unordered_set&) nogil
+        bint operator!=(unordered_set&, unordered_set&) nogil
+        bint operator<(unordered_set&, unordered_set&) nogil
+        bint operator>(unordered_set&, unordered_set&) nogil
+        bint operator<=(unordered_set&, unordered_set&) nogil
+        bint operator>=(unordered_set&, unordered_set&) nogil
+        iterator begin() nogil
+        #const_iterator begin()
+        void clear() nogil
+        size_t count(T&) nogil
+        bint empty() nogil
+        iterator end() nogil
+        #const_iterator end()
+        pair[iterator, iterator] equal_range(T&) nogil
+        #pair[const_iterator, const_iterator] equal_range(T&)
+        void erase(iterator) nogil
+        void erase(iterator, iterator) nogil
+        size_t erase(T&) nogil
+        iterator find(T&) nogil
+        #const_iterator find(T&)
+        pair[iterator, bint] insert(T&) nogil
+        iterator insert(iterator, T&) nogil
+        #void insert(input_iterator, input_iterator)
+        #key_compare key_comp()
+        iterator lower_bound(T&) nogil
+        #const_iterator lower_bound(T&)
+        size_t max_size() nogil
+        reverse_iterator rbegin() nogil
+        #const_reverse_iterator rbegin()
+        reverse_iterator rend() nogil
+        #const_reverse_iterator rend()
+        size_t size() nogil
+        void swap(unordered_set&) nogil
+        iterator upper_bound(T&) nogil
+        #const_iterator upper_bound(T&)
+        #value_compare value_comp()