Replace usage of internal pip function parse_requirements

Starting with pip 10 the `parse_requirement` function was explicitly
moved to an internal namespace. In other words, using it is fraught with
problems since there are no guarantees it won't move again, have its
signature altered, or otherwise. Because of this, replace the use of the
internal function with an equivalent function.

Author: thomasjo

setup.py

@@ -3,7 +3,7 @@
 import sys
 import versioneer
 
-from pip.req import parse_requirements
+from distutils.text_file import TextFile
 from skbuild import setup
 
 
@@ -14,12 +14,13 @@
     history = fp.read().replace('.. :changelog:', '')
 
 
-def _parse_requirements(filename):
-    return [str(ir.req) for ir in parse_requirements(filename, session=False)]
+def parse_requirements(filename):
+    with open(filename, 'r') as file:
+        return TextFile(filename, file).readlines()
 
 
 requirements = []
-dev_requirements = _parse_requirements('requirements-dev.txt')
+dev_requirements = parse_requirements('requirements-dev.txt')
 
 # Require pytest-runner only when running tests
 pytest_runner = (['pytest-runner>=2.0,<3dev']