Dev 3.x flat package (#961)

* Move packages

* Added changelog

* Update examples/echo_bot.py

Co-authored-by: Oleg A. <t0rr@mail.ru>

* Rename `handler` -> `handlers`

* Update __init__.py

Co-authored-by: Oleg A. <t0rr@mail.ru>

tests/test_handler/test_error.py

@@ -0,0 +1,21 @@
+from typing import Any
+
+import pytest
+
+from aiogram.handlers import ErrorHandler
+
+pytestmark = pytest.mark.asyncio
+
+
+class TestErrorHandler:
+    async def test_extensions(self):
+        event = KeyError("kaboom")
+
+        class MyHandler(ErrorHandler):
+            async def handle(self) -> Any:
+                assert self.event == event
+                assert self.exception_name == event.__class__.__name__
+                assert self.exception_message == str(event)
+                return True
+
+        assert await MyHandler(event)