code: add ExceptionInfo.from_exception

The old-style `sys.exc_info()` triplet is redundant nowadays with
`(type(exc), exc, exc.__traceback__)`, and is beginning to get
soft-deprecated in Python 3.12.

Add a nicer API to ExceptionInfo which takes just the exc instead of the
triplet. There are already a few internal uses which benefit.

src/_pytest/python_api.py

@@ -950,11 +950,7 @@ def raises(  # noqa: F811
         try:
             func(*args[1:], **kwargs)
         except expected_exception as e:
-            # We just caught the exception - there is a traceback.
-            assert e.__traceback__ is not None
-            return _pytest._code.ExceptionInfo.from_exc_info(
-                (type(e), e, e.__traceback__)
-            )
+            return _pytest._code.ExceptionInfo.from_exception(e)
     fail(message)