add typing to aiokafka/coordinator/*

Changes

Fixes #

Checklist

[ ] I think the code is well written
[ ] Unit tests for the changes exist
[ ] Documentation reflects the changes
[ ] Add a new news fragment into the CHANGES folder
- name it <issue_id>.<type> (e.g. 588.bugfix)
- if you don't have an issue_id change it to the pr id after creating the PR
- ensure type is one of the following:
- .feature: Signifying a new feature.
- .bugfix: Signifying a bug fix.
- .doc: Signifying a documentation improvement.
- .removal: Signifying a deprecation or removal of public API.
- .misc: A ticket has been closed, but it is not of interest to users.
- Make sure to use full sentences with correct case and punctuation, for example: Fix issue with non-ascii contents in doctest text files.

Codecov Report

Attention: Patch coverage is 96.06987% with 9 lines in your changes missing coverage. Please review.

Project coverage is 95.02%. Comparing base (14aa358) to head (5bf26b2).

Files	Patch %	Lines
...ka/coordinator/assignors/sticky/sticky_assignor.py	93.75%	3 Missing and 2 partials :warning:
...iokafka/coordinator/assignors/sticky/sorted_set.py	85.00%	1 Missing and 2 partials :warning:
aiokafka/coordinator/assignors/roundrobin.py	90.90%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1006 +/- ## ========================================== - Coverage 95.04% 95.02% -0.03% ========================================== Files 114 114 Lines 16906 16970 +64 Branches 2759 2771 +12 ========================================== + Hits 16068 16125 +57 - Misses 493 497 +4 - Partials 345 348 +3 ``` | [Flag](https://app.codecov.io/gh/aio-libs/aiokafka/pull/1006/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aio-libs) | Coverage Δ | | |---|---|---| | [cext](https://app.codecov.io/gh/aio-libs/aiokafka/pull/1006/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aio-libs) | `91.94% <96.06%> (+<0.01%)` | :arrow_up: | | [integration](https://app.codecov.io/gh/aio-libs/aiokafka/pull/1006/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aio-libs) | `94.67% <96.06%> (-0.03%)` | :arrow_down: | | [purepy](https://app.codecov.io/gh/aio-libs/aiokafka/pull/1006/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aio-libs) | `94.45% <96.06%> (-0.06%)` | :arrow_down: | | [unit](https://app.codecov.io/gh/aio-libs/aiokafka/pull/1006/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aio-libs) | `52.86% <96.06%> (+0.14%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aio-libs#carryforward-flags-in-the-pull-request-comment) to find out more.

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aio-libs / aiokafka