Bug#902597:

[Michael Hudson-Doyle]

I cherry picked the attached patch from upstream (which tbh, is a bit
hilarious) to fix the build in Ubuntu.

I also had to add libgmock-dev to build-depends, but i don't know if that's
needed in debian.

From 5c42943e75231b79f4ffcc37a22f0077a2fb0e7d Mon Sep 17 00:00:00 2001
From: Ben Webb <b...@salilab.org>
Date: Tue, 3 Jul 2018 09:51:09 -0700
Subject: [PATCH] Add Python 3.7 compatibility

Compilation of Python wrappers fails with Python 3.7 because
the Python folks changed their C API such that
PyUnicode_AsUTF8AndSize() now returns a const char* rather
than a char*. Add a patch to work around. Relates #4086.
---
 python/google/protobuf/pyext/descriptor.cc            | 2 +-
 python/google/protobuf/pyext/descriptor_containers.cc | 2 +-
 python/google/protobuf/pyext/descriptor_pool.cc       | 2 +-
 python/google/protobuf/pyext/extension_dict.cc        | 2 +-
 python/google/protobuf/pyext/message.cc               | 4 ++--
 5 files changed, 6 insertions(+), 6 deletions(-)

--- a/python/google/protobuf/pyext/descriptor.cc
+++ b/python/google/protobuf/pyext/descriptor.cc
@@ -54,7 +54,7 @@
   #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
 
--- a/python/google/protobuf/pyext/descriptor_containers.cc
+++ b/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
 
--- a/python/google/protobuf/pyext/descriptor_pool.cc
+++ b/python/google/protobuf/pyext/descriptor_pool.cc
@@ -47,7 +47,7 @@
   #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
 
--- a/python/google/protobuf/pyext/message.cc
+++ b/python/google/protobuf/pyext/message.cc
@@ -81,7 +81,7 @@
     (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
@@ -1415,7 +1415,7 @@
     return NULL;
   }
 #else
-  field_name = PyUnicode_AsUTF8AndSize(arg, &size);
+  field_name = const_cast<char*>(PyUnicode_AsUTF8AndSize(arg, &size));
   if (!field_name) {
     return NULL;
   }