Remove dependency on internal pip APIs (#112)

* Remove dependency on internal pip APIs

Fixes #111

* method to parse requirements

@particledecay suggestion

* try updating pip

* pin configparser version

requirements/dev.txt

@@ -11,3 +11,4 @@ setuptools==39.2.0
 twine==1.11.0
 zipp==0.4
 more-itertools<6.0.0; python_version < '3'
+configparser<5.0.0; python_version < '3'

setup.py

@@ -1,13 +1,5 @@
 from setuptools import setup
 
-try:
-    from pip._internal.req import parse_requirements
-except ImportError:
-    from pip.req import parse_requirements
-
-install_req = parse_requirements('requirements/core.txt', session='skip')
-req = [str(ir.req) for ir in install_req]
-
 
 def read_md(filename):
     try:
@@ -17,6 +9,15 @@ def read_md(filename):
         return open(filename).read()
 
 
+def parse_requirements(filename):
+    reqs = []
+    with open(filename, 'r') as f:
+        reqs = f.read().splitlines()
+    if not reqs:
+        raise RuntimeError("Unable to read requirements from '%s'" % filename)
+    return reqs
+
+
 setup(
     name='django_mock_queries',
     version='2.1.4',
@@ -44,5 +45,5 @@ def read_md(filename):
     ],
     keywords='django orm mocking unit-testing tdd',
     packages=['django_mock_queries'],
-    install_requires=req,
+    install_requires=parse_requirements('requirements/core.txt'),
 )