Installation and Bot reject ISO 8601 timestamps with a negative UTC offset or a Z suffix #1989

What happens

Installation, Bot and the token rotation code accept installed_at (and *_token_expires_at) as an ISO 8601 string. Strings with a positive offset parse correctly, but a negative offset or a Z suffix raises ValueError:

from slack_sdk.oauth.installation_store import Installation

Installation(
    user_id="U111",
    installed_at="2021-07-14T03:00:17-05:00",
)
# ValueError: Invalid isoformat string: '2021-07-14T03:00:17-05:00+00:00'

Installation(
    user_id="U111",
    installed_at="2021-07-14T08:00:17Z",
)
# ValueError: Invalid isoformat string: '2021-07-14T08:00:17Z+00:00'  (Python 3.11+)

"2021-07-14T17:00:17+09:00" works, so the result depends on the sign of the offset.

What should happen

Any string datetime.fromisoformat can parse should be accepted. Strings without an offset should keep being treated as UTC.

Root cause

_from_iso_format_to_datetime in slack_sdk/oauth/installation_store/internals.py decides whether a string already has an offset by checking for a + character:

if "+" not in iso_datetime_str:
    iso_datetime_str += "+00:00"
return datetime.fromisoformat(iso_datetime_str)

For -05:00 or Z there is no +, so a second offset is appended and parsing fails. Parsing first and setting UTC only when tzinfo is None fixes it, which matches how naive datetime values are already handled in _timestamp_to_type since #1970.