Add 3.8 and 3.9 to setup.py

[artsiomkaltovich]

Fix project description on pypi.

Should I reflect it in changelog?

[codecov]

Codecov Report

Merging #721 (df67128) into master (5d9af4d) will decrease coverage by 0.47%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #721      +/-   ##
==========================================
- Coverage   97.98%   97.50%   -0.48%     
==========================================
  Files          29       29              
  Lines        5209     5209              
==========================================
- Hits         5104     5079      -25     
- Misses        105      130      +25     

Flag	Coverage Δ
cext	87.23% <ø> (-0.45%)	⬇️
integration	97.50% <ø> (-0.45%)	⬇️
purepy	96.98% <ø> (-0.52%)	⬇️
unit	?

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

Impacted Files	Coverage Δ
aiokafka/client.py	96.62% <0.00%> (-1.54%)	⬇️
aiokafka/consumer/consumer.py	96.71% <0.00%> (-1.20%)	⬇️
aiokafka/producer/sender.py	98.40% <0.00%> (-1.14%)	⬇️
aiokafka/producer/producer.py	97.51% <0.00%> (-1.00%)	⬇️
aiokafka/conn.py	93.19% <0.00%> (-0.91%)	⬇️
aiokafka/producer/message_accumulator.py	98.54% <0.00%> (-0.37%)	⬇️
aiokafka/record/legacy_records.py	98.56% <0.00%> (-0.36%)	⬇️
aiokafka/consumer/fetcher.py	97.02% <0.00%> (-0.32%)	⬇️
aiokafka/consumer/group_coordinator.py	98.89% <0.00%> (-0.14%)	⬇️

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[ods]

Actually there is a problem with 3.8.6+ and 3.9.* due to the bug in wait_for. So declaring at least 3.9 support is arguable.