Support compilation for Python 3.7

Cherry-pick of 51d948a7faed06b9a9ffaa0811efb950a51e5586

direct/src/dcparser/dcPacker.cxx

@@ -768,7 +768,7 @@ pack_object(PyObject *object) {
     pack_int64(PyLong_AsLongLong(object));
 #if PY_MAJOR_VERSION >= 3
   } else if (PyUnicode_Check(object)) {
-    char *buffer;
+    const char *buffer;
     Py_ssize_t length;
     buffer = PyUnicode_AsUTF8AndSize(object, &length);
     if (buffer) {

dtool/src/interrogate/interfaceMakerPythonNative.cxx

@@ -4671,7 +4671,7 @@ write_function_instance(ostream &out, FunctionRemap *remap,
                                       << default_value->_str.size() << ";\n";
           }
         } else {
-          indent(out, indent_level) << "char *" << param_name << "_str = NULL;\n";
+          indent(out, indent_level) << "const char *" << param_name << "_str = NULL;\n";
           indent(out, indent_level) << "Py_ssize_t " << param_name << "_len;\n";
         }
 
@@ -4681,7 +4681,7 @@ write_function_instance(ostream &out, FunctionRemap *remap,
             << param_name << "_str = PyUnicode_AsUTF8AndSize(arg, &"
             << param_name << "_len);\n";
           out << "#else\n"; // NB. PyString_AsStringAndSize also accepts a PyUnicode.
-          indent(out, indent_level) << "if (PyString_AsStringAndSize(arg, &"
+          indent(out, indent_level) << "if (PyString_AsStringAndSize(arg, (char **)&"
             << param_name << "_str, &" << param_name << "_len) == -1) {\n";
           indent(out, indent_level + 2) << param_name << "_str = NULL;\n";
           indent(out, indent_level) << "}\n";

makepanda/makepandacore.py

@@ -2408,6 +2408,8 @@ def SetupBuildEnvironment(compiler):
             if os.path.isdir(pcbsd_inc):
                 SYS_INC_DIRS.append(pcbsd_inc)
 
+        null.close()
+
         # Print out the search paths
         if GetVerbose():
             print("System library search path:")

panda/src/gobj/internalName_ext.cxx

@@ -30,7 +30,7 @@ make(PyUnicodeObject *str) {
   if (!PyUnicode_CHECK_INTERNED(str)) {
     // Not an interned string; don't bother.
     Py_ssize_t len = 0;
-    char *c_str = PyUnicode_AsUTF8AndSize((PyObject *)str, &len);
+    const char *c_str = PyUnicode_AsUTF8AndSize((PyObject *)str, &len);
     if (c_str == NULL) {
       return NULL;
     }
@@ -47,7 +47,7 @@ make(PyUnicodeObject *str) {
 
   } else {
     Py_ssize_t len = 0;
-    char *c_str = PyUnicode_AsUTF8AndSize((PyObject *)str, &len);
+    const char *c_str = PyUnicode_AsUTF8AndSize((PyObject *)str, &len);
     string name(c_str, len);
 
 #else