Use billiard instead of multiprocessing

Multiprocessing lib is causing this library to be unusable in celery
environment, due to celery worker daemon process can't spawn another
child.
While spawning another child is important, because opening
pylokit.Office more than once can cause system exit 77 / 139.
Replacing multiprocessing with billiard enables this library to run in
celery environment.

Author: willycahyadi

setup.cfg

@@ -1,5 +1,5 @@
 [bumpversion]
-current_version = 0.3.3
+current_version = 0.3.4
 commit = True
 tag = True
 tag_name = {new_version}

setup.py

@@ -14,6 +14,7 @@
 requirements = [
     'pylokit==0.8.1',
     'django>=1.8,<1.12',
+    'billiard>=3.5.0.2,<3.6.0',
 ]
 
 test_requirements = [
@@ -22,7 +23,7 @@
 
 setup(
     name='jpt-templated-docs',
-    version='0.3.3',
+    version='0.3.4',
     description=('Generate PDF, MS Word and Excel documents from templates '
                  'in Django.'),
     long_description=readme + '\n\n' + history,

templated_docs/__init__.py

@@ -1,6 +1,6 @@
 # -*- coding: utf-8 -*-
 
-import multiprocessing
+import billiard as multiprocessing
 import os.path
 import re
 from tempfile import NamedTemporaryFile
@@ -24,7 +24,7 @@
 import logging
 log = logging.getLogger(__name__)
 
-__version__ = '0.3.3'
+__version__ = '0.3.4'
 
 
 IMAGES_CONTEXT_KEY = '_templated_docs_imgs'