diff --git a/src/_pytest/assertion/rewrite.py b/src/_pytest/assertion/rewrite.py
index 8c7944593..1f485eb89 100644
--- a/src/_pytest/assertion/rewrite.py
+++ b/src/_pytest/assertion/rewrite.py
@@ -755,8 +755,8 @@ class AssertionRewriter(ast.NodeVisitor):
             import warnings
 
             warnings.warn_explicit(
-                "assertion is always true, perhaps remove parentheses?",
-                PytestWarning,
+                PytestWarning("assertion is always true, perhaps remove parentheses?"),
+                category=None,
                 filename=str(self.module_path),
                 lineno=assert_.lineno,
             )
diff --git a/src/_pytest/config/__init__.py b/src/_pytest/config/__init__.py
index dfee41bdf..993e091f2 100644
--- a/src/_pytest/config/__init__.py
+++ b/src/_pytest/config/__init__.py
@@ -419,11 +419,9 @@ class PytestPluginManager(PluginManager):
                         PYTEST_PLUGINS_FROM_NON_TOP_LEVEL_CONFTEST
                     )
 
-                    from _pytest.warning_types import RemovedInPytest4Warning
-
                     warnings.warn_explicit(
                         PYTEST_PLUGINS_FROM_NON_TOP_LEVEL_CONFTEST,
-                        RemovedInPytest4Warning,
+                        category=None,
                         filename=str(conftestpath),
                         lineno=0,
                     )
@@ -629,7 +627,10 @@ class Config(object):
         if nodeid:
             msg = "{}: {}".format(nodeid, msg)
         warnings.warn_explicit(
-            msg, RemovedInPytest4Warning, filename=filename, lineno=lineno
+            RemovedInPytest4Warning(msg),
+            category=None,
+            filename=filename,
+            lineno=lineno,
         )
         self.hook.pytest_logwarning.call_historic(
             kwargs=dict(
diff --git a/src/_pytest/deprecated.py b/src/_pytest/deprecated.py
index 5e2c58962..dea8bbde8 100644
--- a/src/_pytest/deprecated.py
+++ b/src/_pytest/deprecated.py
@@ -72,7 +72,7 @@ METAFUNC_ADD_CALL = RemovedInPytest4Warning(
     "Please use Metafunc.parametrize instead."
 )
 
-PYTEST_PLUGINS_FROM_NON_TOP_LEVEL_CONFTEST = (
+PYTEST_PLUGINS_FROM_NON_TOP_LEVEL_CONFTEST = RemovedInPytest4Warning(
     "Defining pytest_plugins in a non-top-level conftest is deprecated, "
     "because it affects the entire directory tree in a non-explicit way.\n"
     "Please move it to the top level conftest file instead."
diff --git a/src/_pytest/fixtures.py b/src/_pytest/fixtures.py
index cbfda9a82..068e6814c 100644
--- a/src/_pytest/fixtures.py
+++ b/src/_pytest/fixtures.py
@@ -1284,8 +1284,10 @@ class FixtureManager(object):
 
                 filename, lineno = getfslineno(obj)
                 warnings.warn_explicit(
-                    deprecated.FUNCARG_PREFIX.format(name=name),
-                    RemovedInPytest4Warning,
+                    RemovedInPytest4Warning(
+                        deprecated.FUNCARG_PREFIX.format(name=name)
+                    ),
+                    category=None,
                     filename=str(filename),
                     lineno=lineno + 1,
                 )
diff --git a/src/_pytest/python.py b/src/_pytest/python.py
index 10a3b1ec3..3ce70064e 100644
--- a/src/_pytest/python.py
+++ b/src/_pytest/python.py
@@ -241,8 +241,10 @@ def pytest_pycollect_makeitem(collector, name, obj):
         if not (isfunction(obj) or isfunction(get_real_func(obj))):
             filename, lineno = getfslineno(obj)
             warnings.warn_explicit(
-                message="cannot collect %r because it is not a function." % name,
-                category=PytestWarning,
+                message=PytestWarning(
+                    "cannot collect %r because it is not a function." % name
+                ),
+                category=None,
                 filename=str(filename),
                 lineno=lineno + 1,
             )
diff --git a/testing/deprecated_test.py b/testing/deprecated_test.py
index a74ce662d..fbaca4e30 100644
--- a/testing/deprecated_test.py
+++ b/testing/deprecated_test.py
@@ -202,7 +202,7 @@ def test_pytest_plugins_in_non_top_level_conftest_deprecated(testdir):
     )
     res = testdir.runpytest_subprocess()
     assert res.ret == 0
-    msg = PYTEST_PLUGINS_FROM_NON_TOP_LEVEL_CONFTEST.splitlines()[0]
+    msg = str(PYTEST_PLUGINS_FROM_NON_TOP_LEVEL_CONFTEST).splitlines()[0]
     res.stdout.fnmatch_lines(
         "*subdirectory{sep}conftest.py:0: RemovedInPytest4Warning: {msg}*".format(
             sep=os.sep, msg=msg
@@ -235,7 +235,7 @@ def test_pytest_plugins_in_non_top_level_conftest_deprecated_no_top_level_confte
 
     res = testdir.runpytest_subprocess()
     assert res.ret == 0
-    msg = PYTEST_PLUGINS_FROM_NON_TOP_LEVEL_CONFTEST.splitlines()[0]
+    msg = str(PYTEST_PLUGINS_FROM_NON_TOP_LEVEL_CONFTEST).splitlines()[0]
     res.stdout.fnmatch_lines(
         "*subdirectory{sep}conftest.py:0: RemovedInPytest4Warning: {msg}*".format(
             sep=os.sep, msg=msg
@@ -272,7 +272,7 @@ def test_pytest_plugins_in_non_top_level_conftest_deprecated_no_false_positives(
     )
     res = testdir.runpytest_subprocess()
     assert res.ret == 0
-    msg = PYTEST_PLUGINS_FROM_NON_TOP_LEVEL_CONFTEST.splitlines()[0]
+    msg = str(PYTEST_PLUGINS_FROM_NON_TOP_LEVEL_CONFTEST).splitlines()[0]
     assert msg not in res.stdout.str()