Ensure Monkeypatch setenv and delenv keep bytes keys in Python 2

Fixes the bug described in:

	tox-dev/tox#1025 (comment)

Which is more evident when using `unicode_literals`.

changelog/4056.bugfix.rst

@@ -0,0 +1,4 @@
+Ensure ``MonkeyPatch.setenv`` and ``MonkeyPatch.delenv`` use native strings for environment variable names.
+
+In Python 2, adding ``unicode`` keys to ``os.environ`` causes problems with ``subprocess`` (and possible other modules),
+making this a subtle bug specially susceptible when used with ``from __future__ import unicode_literals``.

src/_pytest/monkeypatch.py

@@ -216,7 +216,7 @@ def setenv(self, name, value, prepend=None):
         value = str(value)
         if prepend and name in os.environ:
             value = value + prepend + os.environ[name]
-        self.setitem(os.environ, name, value)
+        self.setitem(os.environ, str(name), value)
 
     def delenv(self, name, raising=True):
         """ Delete ``name`` from the environment. Raise KeyError if it does
@@ -225,7 +225,7 @@ def delenv(self, name, raising=True):
         If ``raising`` is set to False, no exception will be raised if the
         environment variable is missing.
         """
-        self.delitem(os.environ, name, raising=raising)
+        self.delitem(os.environ, str(name), raising=raising)
 
     def syspath_prepend(self, path):
         """ Prepend ``path`` to ``sys.path`` list of import locations. """

testing/test_monkeypatch.py

@@ -3,6 +3,8 @@
 import sys
 import textwrap
 
+import six
+
 import pytest
 from _pytest.monkeypatch import MonkeyPatch
 
@@ -192,6 +194,40 @@ def test_delenv():
             del os.environ[name]
 
 
+@pytest.mark.skipif(six.PY3, reason="python 2 only bug")
+class TestPython2BytesEnvironKeys(object):
+    """
+    In Python 2, os.environ keys need to be bytes, otherwise this will cause problems with other modules (notably
+    subprocess).
+    """
+
+    VAR_NAME = "PYTEST_INTERNAL_MY_VAR"
+
+    @pytest.fixture(autouse=True)
+    def cleanup(self):
+        self.check_all_environ_keys_are_bytes()
+        yield
+        if self.VAR_NAME in os.environ:
+            del os.environ[self.VAR_NAME]
+
+    def check_all_environ_keys_are_bytes(self):
+        for k in os.environ:
+            assert type(k) is bytes
+
+    def test_setenv_unicode_key(self):
+        monkeypatch = MonkeyPatch()
+        monkeypatch.setenv(unicode(self.VAR_NAME), "2")  # noqa
+        self.check_all_environ_keys_are_bytes()
+
+    def test_delenv_unicode_key(self):
+        os.environ[self.VAR_NAME] = "1"
+        self.check_all_environ_keys_are_bytes()
+        monkeypatch = MonkeyPatch()
+        monkeypatch.delenv(unicode(self.VAR_NAME))  # noqa
+        monkeypatch.undo()
+        self.check_all_environ_keys_are_bytes()
+
+
 def test_setenv_prepend():
     import os