https://github.com/protocolbuffers/protobuf/pull/4862
https://github.com/protocolbuffers/protobuf/commit/0a59054c30e4f0ba10f10acfc1d7f3814c63e1a7

--- /python/google/protobuf/pyext/descriptor.cc
+++ /python/google/protobuf/pyext/descriptor.cc
@@ -56,7 +56,7 @@
   #endif
   #define PyString_AsStringAndSize(ob, charpp, sizep) \
     (PyUnicode_Check(ob)? \
-       ((*(charpp) = PyUnicode_AsUTF8AndSize(ob, (sizep))) == NULL? -1: 0): \
+       ((*(charpp) = const_cast<char*>(PyUnicode_AsUTF8AndSize(ob, (sizep)))) == NULL? -1: 0): \
        PyBytes_AsStringAndSize(ob, (charpp), (sizep)))
 #endif
 
--- /python/google/protobuf/pyext/descriptor_containers.cc
+++ /python/google/protobuf/pyext/descriptor_containers.cc
@@ -66,7 +66,7 @@
   #endif
   #define PyString_AsStringAndSize(ob, charpp, sizep) \
     (PyUnicode_Check(ob)? \
-       ((*(charpp) = PyUnicode_AsUTF8AndSize(ob, (sizep))) == NULL? -1: 0): \
+       ((*(charpp) = const_cast<char*>(PyUnicode_AsUTF8AndSize(ob, (sizep)))) == NULL? -1: 0): \
        PyBytes_AsStringAndSize(ob, (charpp), (sizep)))
 #endif
 
--- /python/google/protobuf/pyext/descriptor_pool.cc
+++ /python/google/protobuf/pyext/descriptor_pool.cc
@@ -48,7 +48,7 @@
   #endif
   #define PyString_AsStringAndSize(ob, charpp, sizep) \
     (PyUnicode_Check(ob)? \
-       ((*(charpp) = PyUnicode_AsUTF8AndSize(ob, (sizep))) == NULL? -1: 0): \
+       ((*(charpp) = const_cast<char*>(PyUnicode_AsUTF8AndSize(ob, (sizep)))) == NULL? -1: 0): \
        PyBytes_AsStringAndSize(ob, (charpp), (sizep)))
 #endif
 
--- /python/google/protobuf/pyext/extension_dict.cc
+++ /python/google/protobuf/pyext/extension_dict.cc
@@ -53,7 +53,7 @@
   #endif
   #define PyString_AsStringAndSize(ob, charpp, sizep) \
     (PyUnicode_Check(ob)? \
-       ((*(charpp) = PyUnicode_AsUTF8AndSize(ob, (sizep))) == NULL? -1: 0): \
+       ((*(charpp) = const_cast<char*>(PyUnicode_AsUTF8AndSize(ob, (sizep)))) == NULL? -1: 0): \
        PyBytes_AsStringAndSize(ob, (charpp), (sizep)))
 #endif
 
--- /python/google/protobuf/pyext/message.cc
+++ /python/google/protobuf/pyext/message.cc
@@ -79,7 +79,7 @@
     (PyUnicode_Check(ob)? PyUnicode_AsUTF8(ob): PyBytes_AsString(ob))
   #define PyString_AsStringAndSize(ob, charpp, sizep) \
     (PyUnicode_Check(ob)? \
-       ((*(charpp) = PyUnicode_AsUTF8AndSize(ob, (sizep))) == NULL? -1: 0): \
+       ((*(charpp) = const_cast<char*>(PyUnicode_AsUTF8AndSize(ob, (sizep)))) == NULL? -1: 0): \
        PyBytes_AsStringAndSize(ob, (charpp), (sizep)))
   #endif
 #endif
@@ -1529,7 +1529,7 @@
     return NULL;
   }
 #else
-  field_name = PyUnicode_AsUTF8AndSize(arg, &size);
+  field_name = const_cast<char*>(PyUnicode_AsUTF8AndSize(arg, &size));
   if (!field_name) {
     return NULL;
   }