Restore skipping tests via raise unittest.SkipTest

Revert "Remove unused code related to `nose` (pytest-dev#13528)"

This reverts commit a620d24 and modifies it adding tests and docs.

Fixes pytest-dev#13895

Author: nicoddemus

changelog/13895.bugfix.rst

@@ -0,0 +1 @@
+Restore support for skipping tests via ``raise unittest.SkipTest``.

src/_pytest/unittest.py

@@ -476,6 +476,16 @@ def pytest_runtest_makereport(item: Item, call: CallInfo[None]) -> None:
             except AttributeError:
                 pass
 
+    # Convert unittest.SkipTest to pytest.skip.
+    # This covers explicit `raise unittest.SkipTest`.
+    unittest = sys.modules.get("unittest")
+    if unittest and call.excinfo and isinstance(call.excinfo.value, unittest.SkipTest):
+        excinfo = call.excinfo
+        call2 = CallInfo[None].from_call(
+            lambda: skip(str(excinfo.value)), call.when
+        )
+        call.excinfo = call2.excinfo
+
 
 def _is_skipped(obj) -> bool:
     """Return True if the given object has been marked with @unittest.skip."""

testing/test_unittest.py

@@ -1093,6 +1093,50 @@ def test_two(self):
     result = pytester.runpytest("-s")
     result.assert_outcomes(passed=2)
 
+def test_skip_setup_class(pytester:Pytester)->None:
+    """
+    Skipping tests in a class by raising unittest.SkipTest in `setUpClass` (#13985).
+    """
+    pytester.makepyfile(
+        """
+        import unittest
+    
+        class Test(unittest.TestCase):
+        
+            @classmethod
+            def setUpClass(cls):
+                raise unittest.SkipTest('Skipping setupclass')
+                
+            def test_foo(self): 
+                assert False
+                
+            def test_bar(self): 
+                assert False
+        """
+    )
+    result = pytester.runpytest()
+    result.assert_outcomes(skipped=2)
+
+
+def test_unittest_skip_function(pytester: Pytester) -> None:
+    """
+    Ensure raising an explicit unittest.SkipTest skips standard pytest functions.
+
+    Support for this is debatable -- technically we only support unittest.SkipTest in TestCase subclasses,
+    but stating this support here in this test because users currently expect this to work,
+    so if we ever break it we at least know we are breaking this use case (#13985).
+    """
+    pytester.makepyfile(
+        """
+        import unittest
+
+        def test_foo(): 
+            raise unittest.SkipTest('Skipping test_foo')
+        """
+    )
+    result = pytester.runpytest()
+    result.assert_outcomes(skipped=1)
+
 
 def test_testcase_handles_init_exceptions(pytester: Pytester) -> None:
     """