Improve the warning message for the implicitly str conversion

changelog/4279.trivial.rst

@@ -0,0 +1 @@
+Improve message and stack level of warnings issued by ``monkeypatch.setenv`` when the value of the environment variable is not a ``str``.

src/_pytest/monkeypatch.py

@@ -230,10 +230,12 @@ def setenv(self, name, value, prepend=None):
         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 print_function
 
 import os
+import re
 import sys
 import textwrap
 
@@ -226,9 +227,10 @@ def test_delenv_unicode_key(self, monkeypatch):
     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)