Fix test to use specifically pickle-unsafe module

The test also includes an assert to confirm the module picked is
actually unsafe, in case it is made safe in the future.
Also include docstring edits suggested by reviews.

pylint/lint/pylinter.py

@@ -402,9 +402,6 @@ def load_plugin_configuration(self) -> None:
                     "bad-plugin-value", args=(modname, module_or_error), line=0
                 )
             elif hasattr(module_or_error, "load_configuration"):
-                # Mypy is not smart enough to realize that we only call
-                # this line if the object has the attr we are looking at.
-                # Hence the one-line type: ignore[union-attr] here.
                 module_or_error.load_configuration(self)  # type: ignore[union-attr]
         # We re-set all the dictionary values to True here to make sure the dict
         # is pickle-able. This is only a problem in multiprocessing/parallel mode.

tests/test_check_parallel.py

@@ -11,8 +11,6 @@
 import argparse
 import multiprocessing
 import os
-import sys
-from pathlib import Path
 
 import dill
 import pytest
@@ -233,21 +231,19 @@ def test_worker_check_single_file_no_checkers(self) -> None:
         assert stats.statement == 18
         assert stats.warning == 0
 
-    def test_linter_with_plugins_is_pickleable(self) -> None:
-        """
-        The linter needs to be pickle-able in order to be passed between workers
-        """
-        dummy_plugin_path = Path(__file__).parent / "regrtest_data" / "dummy_plugin"
-        dummy_plugin_path_str = str(dummy_plugin_path.absolute())
-        sys.path.append(dummy_plugin_path_str)
+    def test_linter_with_unpickleable_plugins_is_pickleable(self) -> None:
+        """The linter needs to be pickle-able in order to be passed between workers"""
         linter = PyLinter(reporter=Reporter())
-        print(linter._dynamic_plugins)
+        # We load an extension that we know is not pickle-safe
+        linter.load_plugin_modules(["pylint.extensions.overlapping_exceptions"])
+        with pytest.raises(KeyError):
+            dill.dumps(linter)
+        # And expect this call to make it pickle-able
+        linter.load_plugin_configuration()
         try:
             dill.dumps(linter)
-        except dill.PicklingError:
-            assert False, "Loading plugins caused an un-pickle-able linter"
-        finally:
-            sys.path.remove(dummy_plugin_path_str)
+        except KeyError:
+            assert False, "Cannot pickle linter when using non-pickleable plugin"
 
     def test_worker_check_sequential_checker(self) -> None:
         """Same as test_worker_check_single_file_no_checkers with SequentialTestChecker."""