[EventHub] typing for class variables

[JJHWAN]

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Some class variables in eventhub has older version of typing.

Such as
`class EventProcessorMixin(object):

_eventhub_client = (
    None
)  # type: Optional[Union[EventHubConsumerClient, EventHubConsumerClientAsync]]
_consumer_group = ""  # type: str
_owner_level = None  # type: Optional[int]
_prefetch = None  # type: Optional[int]
_track_last_enqueued_event_properties = False  # type: bool
_initial_event_position_inclusive = {}  # type: Union[bool, Dict[str, bool]]
_initial_event_position = (
    {}
)  # type: Union[int, str, datetime, Dict[str, Union[int, str, datetime]]]

`
From this typing link

It seems to be better to change typing to
Example :
from
_consumer_group = "" # type: str
to
_consumer_group: str = ""

I think it may be applied to other directories too

[kristapratico]

Hi @JJHWAN thanks for your feedback, our team will take a look.

[swathipil]

@JJHWAN - Thanks for submitting a PR to address this! #26208 (review)

Just reviewed and can take another look after my comments have been addressed.

[swathipil]

pr merged