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_dispatcher/test_handler/test_poll.py

@@ -1,33 +0,0 @@
-from typing import Any
-
-import pytest
-
-from aiogram.dispatcher.handler import PollHandler
-from aiogram.types import Poll, PollOption
-
-pytestmark = pytest.mark.asyncio
-
-
-class TestShippingQueryHandler:
-    async def test_attributes_aliases(self):
-        event = Poll(
-            id="query",
-            question="Q?",
-            options=[PollOption(text="A1", voter_count=1)],
-            is_closed=True,
-            is_anonymous=False,
-            type="quiz",
-            allows_multiple_answers=False,
-            total_voter_count=0,
-            correct_option_id=0,
-        )
-
-        class MyHandler(PollHandler):
-            async def handle(self) -> Any:
-                assert self.event == event
-                assert self.question == self.event.question
-                assert self.options == self.event.options
-
-                return True
-
-        assert await MyHandler(event)