Improve the warning message for the implicitly str conversion

Signed-off-by: William Jamir Silva <williamjamir@gmail.com>

src/_pytest/monkeypatch.py

@@ -230,10 +230,12 @@ class MonkeyPatch(object):
         if not isinstance(value, str):
             warnings.warn(
                 pytest.PytestWarning(
-                    "Environment variable value {!r} should be str, converted to str implicitly".format(
-                        value
+                    "Value of environment variable {name} type should be str, but got "
+                    "{value!r} (type: {type}); converted to str implicitly".format(
+                        name=name, value=value, type=type(value).__name__
                     )
-                )
+                ),
+                stacklevel=2,
             )
             value = str(value)
         if prepend and name in os.environ:

testing/test_monkeypatch.py

@@ -3,6 +3,7 @@ from __future__ import division
 from __future__ import print_function
 
 import os
+import re
 import sys
 import textwrap
 
@@ -226,9 +227,10 @@ class TestEnvironWarnings(object):
     def test_setenv_non_str_warning(self, monkeypatch):
         value = 2
         msg = (
-            "Environment variable value {!r} should be str, converted to str implicitly"
+            "Value of environment variable PYTEST_INTERNAL_MY_VAR type should be str, "
+            "but got 2 (type: int); converted to str implicitly"
         )
-        with pytest.warns(pytest.PytestWarning, match=msg.format(value)):
+        with pytest.warns(pytest.PytestWarning, match=re.escape(msg)):
             monkeypatch.setenv(str(self.VAR_NAME), value)
 
 
@@ -337,7 +339,7 @@ def test_importerror(testdir):
     )
     testdir.tmpdir.join("test_importerror.py").write(
         textwrap.dedent(
-            """\
+            r"""\
         def test_importerror(monkeypatch):
             monkeypatch.setattr('package.a.x', 2)
     """