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.

doc/en/how-to/assert.rst

@@ -98,7 +98,7 @@ and if you need to have access to the actual exception info you may use:
             f()
         assert "maximum recursion" in str(excinfo.value)
 
-``excinfo`` is an ``ExceptionInfo`` instance, which is a wrapper around
+``excinfo`` is an :class:`~pytest.ExceptionInfo` instance, which is a wrapper around
 the actual exception raised.  The main attributes of interest are
 ``.type``, ``.value`` and ``.traceback``.
 

doc/en/reference/reference.rst

@@ -793,7 +793,7 @@ Config
 ExceptionInfo
 ~~~~~~~~~~~~~
 
-.. autoclass:: _pytest._code.ExceptionInfo
+.. autoclass:: pytest.ExceptionInfo()
     :members: