code: export ExceptionInfo for typing purposes

This type is most prominent in `pytest.raises` and we should allow to
refer to it by a public name.

The type is not in a perfectly "exposable" state. In particular:

- The `traceback` property with type `Traceback` which is derived from
  the `py.code` API and exposes a bunch more types transitively. This
  stuff is *not* exported and probably won't be.

- The `getrepr` method which probably should be private.

But they're already used in the wild so no point in just hiding them
now.

The __init__ API is hidden -- the public API for this are the `from_*`
classmethods.

src/_pytest/_code/code.py

@@ -42,6 +42,7 @@ from _pytest._io.saferepr import safeformat
 from _pytest._io.saferepr import saferepr
 from _pytest.compat import final
 from _pytest.compat import get_real_func
+from _pytest.deprecated import check_ispytest
 from _pytest.pathlib import absolutepath
 from _pytest.pathlib import bestrelpath
 
@@ -440,15 +441,28 @@ E = TypeVar("E", bound=BaseException, covariant=True)
 
 
 @final
-@attr.s(repr=False)
+@attr.s(repr=False, init=False)
 class ExceptionInfo(Generic[E]):
     """Wraps sys.exc_info() objects and offers help for navigating the traceback."""
 
     _assert_start_repr = "AssertionError('assert "
 
     _excinfo = attr.ib(type=Optional[Tuple[Type["E"], "E", TracebackType]])
-    _striptext = attr.ib(type=str, default="")
-    _traceback = attr.ib(type=Optional[Traceback], default=None)
+    _striptext = attr.ib(type=str)
+    _traceback = attr.ib(type=Optional[Traceback])
+
+    def __init__(
+        self,
+        excinfo: Optional[Tuple[Type["E"], "E", TracebackType]],
+        striptext: str = "",
+        traceback: Optional[Traceback] = None,
+        *,
+        _ispytest: bool = False,
+    ) -> None:
+        check_ispytest(_ispytest)
+        self._excinfo = excinfo
+        self._striptext = striptext
+        self._traceback = traceback
 
     @classmethod
     def from_exc_info(
@@ -475,7 +489,7 @@ class ExceptionInfo(Generic[E]):
             if exprinfo and exprinfo.startswith(cls._assert_start_repr):
                 _striptext = "AssertionError: "
 
-        return cls(exc_info, _striptext)
+        return cls(exc_info, _striptext, _ispytest=True)
 
     @classmethod
     def from_current(
@@ -502,7 +516,7 @@ class ExceptionInfo(Generic[E]):
     @classmethod
     def for_later(cls) -> "ExceptionInfo[E]":
         """Return an unfilled ExceptionInfo."""
-        return cls(None)
+        return cls(None, _ispytest=True)
 
     def fill_unfilled(self, exc_info: Tuple[Type[E], E, TracebackType]) -> None:
         """Fill an unfilled ExceptionInfo created with ``for_later()``."""
@@ -922,7 +936,7 @@ class FormattedExcinfo:
             if e.__cause__ is not None and self.chain:
                 e = e.__cause__
                 excinfo_ = (
-                    ExceptionInfo((type(e), e, e.__traceback__))
+                    ExceptionInfo.from_exc_info((type(e), e, e.__traceback__))
                     if e.__traceback__
                     else None
                 )
@@ -932,7 +946,7 @@ class FormattedExcinfo:
             ):
                 e = e.__context__
                 excinfo_ = (
-                    ExceptionInfo((type(e), e, e.__traceback__))
+                    ExceptionInfo.from_exc_info((type(e), e, e.__traceback__))
                     if e.__traceback__
                     else None
                 )