Fix deduce_helpful_msg in python3

Add a test and changelog for #4127

news/4127.feature

@@ -0,0 +1 @@
+Improve the error message for the common ``pip install ./requirements.txt`` case.

pip/req/req_install.py

@@ -1069,7 +1069,7 @@ def deduce_helpful_msg(req):
         try:
             with open(req, 'r') as fp:
                 # parse first line only
-                parse_requirements(fp.read()).next()
+                next(parse_requirements(fp.read()))
                 msg += " The argument you provided " + \
                     "(%s) appears to be a" % (req) + \
                     " requirements file. If that is the" + \

tests/unit/test_req.py

@@ -524,6 +524,18 @@ class TestInstallRequirement(object):
         assert "Invalid requirement" in err_msg
         assert "\nTraceback " in err_msg
 
+    def test_requirement_file(self):
+        req_file_path = os.path.join(self.tempdir, 'test.txt')
+        with open(req_file_path, 'w') as req_file:
+            req_file.write('pip\nsetuptools')
+        with pytest.raises(InstallationError) as e:
+            InstallRequirement.from_line(req_file_path)
+        err_msg = e.value.args[0]
+        assert "Invalid requirement" in err_msg
+        assert "It looks like a path. It does exist." in err_msg
+        assert "appears to be a requirements file." in err_msg
+        assert "If that is the case, use the '-r' flag to install" in err_msg
+
 
 def test_requirements_data_structure_keeps_order():
     requirements = Requirements()