Fix #72: Add support for python_implementation marker

This is legacy from setuptools, supported from 0.7 to 20.1.1
(inclusive).

packaging/markers.py

@@ -78,9 +78,14 @@ class Value(Node):
     L("platform.version") |  # PEP-345
     L("platform.machine") |  # PEP-345
     L("platform.python_implementation") |  # PEP-345
+    L("python_implementation") |  # undocumented setuptools legacy
     L("extra")
 )
-VARIABLE.setParseAction(lambda s, l, t: Variable(t[0].replace('.', '_')))
+ALIASES = {
+    'python_implementation': 'platform_python_implementation'
+}
+VARIABLE.setParseAction(lambda s, l, t:
+                        Variable(ALIASES.get(t[0], t[0].replace('.', '_'))))
 
 VERSION_CMP = (
     L("===") |

tests/test_markers.py

@@ -30,6 +30,10 @@
     "platform.python_implementation",
 ]
 
+SETUPTOOLS_VARIABLES = [
+    "python_implementation",
+]
+
 OPERATORS = [
     "===", "==", ">=", "<=", "!=", "~=", ">", "<", "in", "not in",
 ]
@@ -346,3 +350,21 @@ def test_evaluate_pep345_markers(self, marker_string, environment,
                                      expected):
         args = [] if environment is None else [environment]
         assert Marker(marker_string).evaluate(*args) == expected
+
+    @pytest.mark.parametrize(
+        "marker_string",
+        [
+            "{0} {1} {2!r}".format(*i)
+            for i in itertools.product(SETUPTOOLS_VARIABLES, OPERATORS, VALUES)
+        ] + [
+            "{2!r} {1} {0}".format(*i)
+            for i in itertools.product(SETUPTOOLS_VARIABLES, OPERATORS, VALUES)
+        ],
+    )
+    def test_parses_setuptools_legacy_valid(self, marker_string):
+        Marker(marker_string)
+
+    def test_evaluate_setuptools_legacy_markers(self):
+        marker_string = "python_implementation=='Jython'"
+        args = [{"platform_python_implementation": "CPython"}]
+        assert Marker(marker_string).evaluate(*args) is False