Enable pip install -e.[dev] to install development requirements

Author: adamtheturtle

setup.py

@@ -1,21 +1,36 @@
 from setuptools import setup
 from codecs import open
 from os import path
+from pathlib import Path
+from typing import List
 
 from pip_check_reqs import __version__
 
 here = path.abspath(path.dirname(__file__))
 
+
+def _get_dependencies(requirements_file: Path) -> List[str]:
+    """
+    Return requirements from a requirements file.
+    This expects a requirements file with no ``--find-links`` lines.
+    """
+    lines = requirements_file.read_text().strip().split('\n')
+    return [line for line in lines if not line.startswith('#')]
+
+
 with open(path.join(here, 'README.rst'), encoding='utf-8') as f:
     long_description = f.read()
 
 with open(path.join(here, 'CHANGELOG.rst'), encoding='utf-8') as f:
     long_description += f.read()
 
-# This is not usual, but this project needs both install_requires
-# and requirements.txt and we'd like to not duplicate them
-with open(path.join(here, 'requirements.txt'), encoding='utf-8') as f:
-    requirements = [s.strip() for s in f.readlines()]
+INSTALL_REQUIRES = _get_dependencies(
+    requirements_file=Path('requirements.txt'),
+)
+
+DEV_REQUIRES = _get_dependencies(
+    requirements_file=Path('test-requirements.txt'),
+)
 
 setup(
     name='pip_check_reqs',
@@ -44,5 +59,6 @@
             'pip-extra-reqs=pip_check_reqs.find_extra_reqs:main',
         ],
     },
-    install_requires=requirements,
+    install_requires=INSTALL_REQUIRES,
+    extras_require={'dev': DEV_REQUIRES},
 )