Remove cdecimal dependency

It is no more needed as it is standard under Python3

doc/topics/install.rst

@@ -22,7 +22,6 @@ Prerequisites
       (http://github.com/chtd/psycopg2cffi)
     * Optional: pydot (http://code.google.com/p/pydot/)
     * Optional: sphinx (http://sphinx.pocoo.org/)
-    * Optional: cdecimal (http://www.bytereef.org/mpdecimal/index.html)
     * Optional: python-Levenshtein
       (http://github.com/miohtama/python-Levenshtein)
     * Optional: bcrypt (https://github.com/pyca/bcrypt)

setup.py

@@ -108,7 +108,6 @@ setup(name=name,
     extras_require={
         'PostgreSQL': pg_require,
         'graphviz': ['pydot'],
-        'cdecimal': ['cdecimal'],
         'Levenshtein': ['python-Levenshtein'],
         'BCrypt': ['bcrypt'],
         'html2text': ['html2text'],

trytond/tests/test_exportdata.py

@@ -1,14 +1,6 @@
 # -*- coding: utf-8 -*-
 # This file is part of Tryton.  The COPYRIGHT file at the top level of
 # this repository contains the full copyright notices and license terms.
-import sys
-try:
-    import cdecimal
-    if 'decimal' not in sys.modules:
-        sys.modules['decimal'] = cdecimal
-except ImportError:
-    import decimal
-    sys.modules['cdecimal'] = decimal
 import unittest
 from decimal import Decimal
 import datetime