Use pip-api instead of pip's internal APIs (#59)

* Don't use pip's vendored packaging

* Use pip_api instead of pip's internal APIs

* Add a python_requires

hashin.py

@@ -13,8 +13,8 @@
 import json
 from itertools import chain
 
-import pip
-from pip._vendor.packaging.version import parse
+import pip_api
+from packaging.version import parse
 
 if sys.version_info >= (3,):
     from urllib.request import urlopen
@@ -72,7 +72,7 @@
 )
 
 
-major_pip_version = int(pip.__version__.split('.')[0])
+major_pip_version = int(pip_api.version().split('.')[0])
 if major_pip_version < 8:
     raise ImportError(
         'hashin only works with pip 8.x or greater'
@@ -363,7 +363,7 @@ def get_releases_hashes(releases, algorithm, verbose=False):
             elif verbose:
                 _verbose('  Re-using', filename)
 
-            found['hash'] = pip.commands.hash._hash_of_file(filename, algorithm)
+            found['hash'] = pip_api.hash(filename, algorithm)
         if verbose:
             _verbose('  Hash', found['hash'])
         yield {

setup.py

@@ -25,6 +25,8 @@
         },
     url='https://github.com/peterbe/hashin',
     include_package_data=True,
+    python_requires='>=2.7,!=3.0,!=3.1,!=3.2,!=3.3',
+    install_requires=['packaging', 'pip-api'],
     tests_require=['pytest', 'mock'],
     setup_requires=['pytest-runner'],
     classifiers=[