fix(bot,dispatcher): do not use _MainThread event loop

[uwinx]

Description

on ::Bot, ::Dispatcher initializators

Fixes #365

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• ~Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Test:

The following should work

from aiogram import Bot, Dispatcher, types

bot = Bot(TOKEN)
dp = Dispatcher(bot=bot)


@dp.message_handler()
async def echo_processor(event: types.Message):
    await event.reply(event.html_text, parse_mode=types.ParseMode.HTML)


async def main():
    await bot.send_message(CHAT_ID, "Hi!")
    try:
        await dp.start_polling()
    finally:
        await bot.close()
        await dp.wait_closed()


if __name__ == '__main__':
    import asyncio
    asyncio.run(main())

[uwinx]

I'd love to get rid of loop attribute from everywhere possible, but for now we need to store it for the sake of backward compatibility 😬