fix(BaseBot): remove __del__ method from BaseBot

[uwinx]

implement "lazy" session property getter and new get_new_session for BaseBot

Fixes #314 #305

Type of change

• Bug fix
• New feature
• ? Breaking change

How Has This Been Tested?

if __name__ == '__main__':
    import asyncio

    async def main():
        from aiogram.bot.bot import Bot

        bot = Bot(">.<")
        await bot.send_message(124191486, "sup!")
        await bot.close()
        await bot.send_message(124191486, "sup! 2")
        await bot.close()

    asyncio.run(main())

Test Configuration:

• Python version: 3.8

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