Remove outdated information from the doc

[vmaurin]

aiokafka is not dependent anymore on kafka-python

Changes

Fixes #1032

Checklist

• [x] I think the code is well written
• [ ] Unit tests for the changes exist
• [x] 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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.02%. Comparing base (2f976d0) to head (0d91dbd).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1033 +/- ## ======================================= Coverage 95.02% 95.02% ======================================= Files 114 114 Lines 16970 16970 Branches 2771 2771 ======================================= Hits 16125 16125 Misses 497 497 Partials 348 348 ``` | [Flag](https://app.codecov.io/gh/aio-libs/aiokafka/pull/1033/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/1033/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aio-libs) | `91.94% <ø> (ø)` | | | [integration](https://app.codecov.io/gh/aio-libs/aiokafka/pull/1033/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aio-libs) | `94.66% <ø> (-0.02%)` | :arrow_down: | | [purepy](https://app.codecov.io/gh/aio-libs/aiokafka/pull/1033/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aio-libs) | `94.49% <ø> (ø)` | | | [unit](https://app.codecov.io/gh/aio-libs/aiokafka/pull/1033/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aio-libs) | `52.88% <ø> (ø)` | | 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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[ods]

Thank you for the contribution! Added with other doc fixes in https://github.com/aio-libs/aiokafka/pull/1034