python: allow to customize Python namespaces

- extra methods through PYOPENCV_EXTRA_METHODS_<NS_UPPER> macros
- extra constants through PYOPENCV_EXTRA_CONSTANTS_<NS_UPPER> macros

modules/python/src2/gen2.py

@@ -976,6 +976,8 @@ class PythonWrapperGenerator(object):
             if func.isconstructor:
                 continue
             self.code_ns_reg.write(func.get_tab_entry())
+        custom_entries_macro = 'PYOPENCV_EXTRA_METHODS_{}'.format(wname.upper())
+        self.code_ns_reg.write('#ifdef {}\n    {}\n#endif\n'.format(custom_entries_macro, custom_entries_macro))
         self.code_ns_reg.write('    {NULL, NULL}\n};\n\n')
 
         self.code_ns_reg.write('static ConstDef consts_%s[] = {\n'%wname)
@@ -984,6 +986,8 @@ class PythonWrapperGenerator(object):
             compat_name = re.sub(r"([a-z])([A-Z])", r"\1_\2", name).upper()
             if name != compat_name:
                 self.code_ns_reg.write('    {"%s", static_cast<long>(%s)},\n'%(compat_name, cname))
+        custom_entries_macro = 'PYOPENCV_EXTRA_CONSTANTS_{}'.format(wname.upper())
+        self.code_ns_reg.write('#ifdef {}\n    {}\n#endif\n'.format(custom_entries_macro, custom_entries_macro))
         self.code_ns_reg.write('    {NULL, 0}\n};\n\n')
 
     def gen_enum_reg(self, enum_name):