Removed usage of deprecated pkg_resources

Define version instead in polymorphic/__init__.py file.

polymorphic/__init__.py

@@ -6,9 +6,4 @@
 Please see LICENSE and AUTHORS for more information.
 """
 
-import pkg_resources
-
-try:
-    __version__ = pkg_resources.require("django-polymorphic")[0].version
-except pkg_resources.DistributionNotFound:
-    __version__ = None  # for RTD among others
+__version__ = "3.1.0"

setup.cfg

@@ -1,6 +1,5 @@
 [metadata]
 name = django-polymorphic
-version = 3.1.0
 description = Seamless polymorphic inheritance for Django models
 long_description = file:README.rst
 author = Bert Constantin

setup.py

@@ -1,5 +1,6 @@
 #!/usr/bin/env python
 import os
+import re
 import sys
 
 from setuptools import find_packages, setup
@@ -19,4 +20,13 @@
 
 from setuptools import setup
 
-setup()
+def get_version(package):
+    """
+    Return package version as listed in `__version__` in `init.py`.
+    """
+    init_py = open(os.path.join(package, "__init__.py")).read()
+    return re.search("__version__ = ['\"]([^'\"]+)['\"]", init_py).group(1)
+
+setup(
+    version=get_version("polymorphic"),
+)