Corrected the type annotation for the exception handler callback (#109)

Fixes #97.

Also used Python formatting for all code blocks in the README.

CHANGES.rst

@@ -3,6 +3,11 @@ Version history
 
 This library adheres to `Semantic Versioning 2.0 <http://semver.org/>`_.
 
+**UNRELEASED**
+
+- Corrected the type annotation of the exception handler callback to accept a
+  ``BaseExceptionGroup`` instead of ``BaseException``
+
 **1.2.0**
 
 - Added special monkeypatching if `Apport <https://github.com/canonical/apport>`_ has

README.rst

@@ -54,7 +54,7 @@ containing more matching exceptions.
 
 Thus, the following Python 3.11+ code:
 
-.. code-block:: python3
+.. code-block:: python
 
     try:
         ...
@@ -66,15 +66,15 @@ Thus, the following Python 3.11+ code:
 
 would be written with this backport like this:
 
-.. code-block:: python3
+.. code-block:: python
 
-    from exceptiongroup import ExceptionGroup, catch
+    from exceptiongroup import BaseExceptionGroup, catch
 
-    def value_key_err_handler(excgroup: ExceptionGroup) -> None:
+    def value_key_err_handler(excgroup: BaseExceptionGroup) -> None:
         for exc in excgroup.exceptions:
             print('Caught exception:', type(exc))
 
-    def runtime_err_handler(exc: ExceptionGroup) -> None:
+    def runtime_err_handler(exc: BaseExceptionGroup) -> None:
         print('Caught runtime error')
 
     with catch({
@@ -91,7 +91,9 @@ Suppressing exceptions
 
 This library contains a backport of the ``contextlib.suppress()`` context manager from
 Python 3.12.1. It allows you to selectively ignore certain exceptions, even when they're
-inside exception groups::
+inside exception groups:
+
+.. code-block:: python
 
     from exceptiongroup import suppress
 

src/exceptiongroup/_catch.py

@@ -11,7 +11,7 @@
     from ._exceptions import BaseExceptionGroup
 
 if TYPE_CHECKING:
-    _Handler = Callable[[BaseException], Any]
+    _Handler = Callable[[BaseExceptionGroup[Any]], Any]
 
 
 class _Catcher: