Issue warning if Monkeypatch.setenv/delenv receive non-strings in Python 2

Fixes the bug described in:

	https://github.com/tox-dev/tox/pull/1025#discussion_r221273830

Which is more evident when using `unicode_literals`.

src/_pytest/monkeypatch.py

@@ -4,9 +4,12 @@ from __future__ import absolute_import, division, print_function
 import os
 import sys
 import re
+import warnings
 from contextlib import contextmanager
 
 import six
+
+import pytest
 from _pytest.fixtures import fixture
 
 RE_IMPORT_ERROR_NAME = re.compile("^No module named (.*)$")
@@ -209,6 +212,15 @@ class MonkeyPatch(object):
             self._setitem.append((dic, name, dic.get(name, notset)))
             del dic[name]
 
+    def _warn_if_env_name_is_not_str(self, name):
+        """On Python 2, warn if the given environment variable name is not a native str (#4056)"""
+        if six.PY2 and not isinstance(name, str):
+            warnings.warn(
+                pytest.PytestWarning(
+                    "Environment variable name {!r} should be str".format(name)
+                )
+            )
+
     def setenv(self, name, value, prepend=None):
         """ Set environment variable ``name`` to ``value``.  If ``prepend``
         is a character, read the current environment variable value
@@ -216,6 +228,7 @@ class MonkeyPatch(object):
         value = str(value)
         if prepend and name in os.environ:
             value = value + prepend + os.environ[name]
+        self._warn_if_env_name_is_not_str(name)
         self.setitem(os.environ, name, value)
 
     def delenv(self, name, raising=True):
@@ -225,6 +238,7 @@ class MonkeyPatch(object):
         If ``raising`` is set to False, no exception will be raised if the
         environment variable is missing.
         """
+        self._warn_if_env_name_is_not_str(name)
         self.delitem(os.environ, name, raising=raising)
 
     def syspath_prepend(self, path):