setup.py: apply primogen version 20181203.

setup.py

@@ -1,14 +1,33 @@
 #!/usr/bin/env python
 # -*- coding: utf-8 -*-
 
-import pip.download
-
-from pip.req import parse_requirements
+import pip
 
 from setuptools import find_packages, setup
 
+if int(pip.__version__.split('.')[0]) >= 10:
+    from pip._internal import download as pip_download
+    from pip._internal.req import parse_requirements
+
+else:
+    from pip import download as pip_download
+    from pip.req import parse_requirements
+
 exec(open('steenzout/serialization/json/metadata.py').read())
 
+
+def requirements(requirements_file):
+    """Return packages mentioned in the given file.
+    Args:
+        requirements_file (str): path to the requirements file to be parsed.
+    Returns:
+        (list): 3rd-party package dependencies contained in the file.
+    """
+    return [
+        str(pkg.req) for pkg in parse_requirements(
+            requirements_file, session=pip_download.PipSession()) if pkg.req is not None]
+
+
 setup(
     name='steenzout.serialization.json',
     version=__version__,
@@ -21,11 +40,7 @@
     namespace_packages=['steenzout'],
     packages=find_packages(exclude=('*.tests', '*.tests.*', 'tests.*', 'tests')),
     package_data={'': ['LICENSE', 'NOTICE.md']},
-    install_requires=[
-        str(pkg.req) for pkg in parse_requirements(
-            'requirements.txt', session=pip.download.PipSession())],
-    tests_require=[
-        str(pkg.req) for pkg in parse_requirements(
-            'requirements-test.txt', session=pip.download.PipSession())],
+    install_requires=requirements('requirements.txt'),
+    tests_require=requirements('requirements-test.txt'),
     license=__license__,
     classifiers=__classifiers__)