Bugfix: prevent AttributeError (#685)

[moio]

Changes

Fixes #685

__aenter__ is supposed to return the object that populates the instance variable in async with Constructor(params) as instance:

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

Merging #686 (76e07ba) into master (f4ac354) will decrease coverage by 0.03%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #686      +/-   ##
==========================================
- Coverage   97.99%   97.95%   -0.04%     
==========================================
  Files          29       29              
  Lines        5193     5194       +1     
==========================================
- Hits         5089     5088       -1     
- Misses        104      106       +2     

Flag	Coverage Δ
cext	87.63% <100.00%> (-0.08%)	:arrow_down:
integration	97.92% <100.00%> (-0.04%)	:arrow_down:
purepy	97.47% <100.00%> (+0.03%)	:arrow_up:
unit	51.44% <0.00%> (-0.01%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
aiokafka/producer/producer.py	98.50% <100.00%> (+<0.01%)	:arrow_up:
aiokafka/consumer/fetcher.py	97.17% <0.00%> (-0.32%)	:arrow_down:

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[moio]

Can this PR get a review?