Revert "Let context-managers for raises and warns handle unknown keyword arguments"

This reverts commit dfe54cd82f.

The idea in the commit was to simplify the code by removing the check
and instead letting it TypeError which has the same effect.

However this type error is caught by mypy, and rather than ignoring the
error we think it's better and clearer to go back to the previous
explicit check.

src/_pytest/python_api.py

@@ -653,9 +653,12 @@ def raises(expected_exception, *args, match=None, **kwargs):
     message = "DID NOT RAISE {}".format(expected_exception)
 
     if not args:
-        return RaisesContext(
-            expected_exception, message=message, match_expr=match, **kwargs
-        )
+        if kwargs:
+            msg = "Unexpected keyword arguments passed to pytest.raises: "
+            msg += ", ".join(sorted(kwargs))
+            msg += "\nUse context-manager form instead?"
+            raise TypeError(msg)
+        return RaisesContext(expected_exception, message, match)
     else:
         func = args[0]
         if not callable(func):

src/_pytest/recwarn.py

@@ -76,7 +76,12 @@ def warns(expected_warning, *args, match=None, **kwargs):
     """
     __tracebackhide__ = True
     if not args:
-        return WarningsChecker(expected_warning, match_expr=match, **kwargs)
+        if kwargs:
+            msg = "Unexpected keyword arguments passed to pytest.warns: "
+            msg += ", ".join(sorted(kwargs))
+            msg += "\nUse context-manager form instead?"
+            raise TypeError(msg)
+        return WarningsChecker(expected_warning, match_expr=match)
     else:
         func = args[0]
         if not callable(func):