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/__init__.py

@@ -2,6 +2,7 @@
 """pytest: unit and functional testing with Python."""
 from . import collect
 from _pytest import __version__
+from _pytest._code import ExceptionInfo
 from _pytest.assertion import register_assert_rewrite
 from _pytest.cacheprovider import Cache
 from _pytest.capture import CaptureFixture
@@ -79,6 +80,7 @@ __all__ = [
     "console_main",
     "deprecated_call",
     "exit",
+    "ExceptionInfo",
     "ExitCode",
     "fail",
     "File",