Use sys.executable if it matches the requested version

tests/languages/python_test.py

@@ -2,6 +2,10 @@ from __future__ import absolute_import
 from __future__ import unicode_literals
 
 import os.path
+import sys
+
+import mock
+import pytest
 
 from pre_commit.languages import python
 
@@ -16,3 +20,15 @@ def test_norm_version_expanduser():
         expected_path = home + '/.pyenv/versions/3.4.3/bin/python'
     result = python.norm_version(path)
     assert result == expected_path
+
+
+@pytest.mark.parametrize('v', ('python3.6', 'python3', 'python'))
+def test_sys_executable_matches(v):
+    with mock.patch.object(sys, 'version_info', (3, 6, 7)):
+        assert python._sys_executable_matches(v)
+
+
+@pytest.mark.parametrize('v', ('notpython', 'python3.x'))
+def test_sys_executable_matches_does_not_match(v):
+    with mock.patch.object(sys, 'version_info', (3, 6, 7)):
+        assert not python._sys_executable_matches(v)