Allow multiple threads in ProducerConsumer with python <= 3.7

[christian-monch]

The new runner implementation allows for threads in ProducerConsumer in python versions <= 3.7 because we
do not use asyncio any longer in the runner

[qlty-cloud-legacy]

Code Climate has analyzed commit 01dec66 and detected 1 issue on this pull request.

Here's the issue category breakdown:

Category	Count
Security	1

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[codecov]

Codecov Report

Merging #6371 (01dec66) into master (c666b36) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #6371      +/-   ##
==========================================
- Coverage   89.84%   89.82%   -0.02%     
==========================================
  Files         331      331              
  Lines       43196    43230      +34     
==========================================
+ Hits        38810    38833      +23     
- Misses       4386     4397      +11     

Impacted Files	Coverage Δ
datalad/support/parallel.py	90.55% <ø> (-4.84%)	⬇️
datalad/support/tests/test_parallel.py	94.85% <ø> (-0.08%)	⬇️
datalad/local/tests/test_download_url.py	98.13% <0.00%> (-1.87%)	⬇️
datalad/utils.py	85.25% <0.00%> (+0.08%)	⬆️
datalad/distributed/tests/test_ria_basics.py	97.91% <0.00%> (+0.29%)	⬆️
datalad/distributed/ora_remote.py	31.78% <0.00%> (+0.33%)	⬆️

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[christian-monch]

We are about to have 3.7 as minimal supported version in master

It makes sense then for the 3.7 versions. And it removes code that is no longer needed from 3.8 onwards

[yarikoptic]

Thank you @christian-monch !
looks good to me and I guess could even go against maint (I would have liked it since I kept running into this warning for quite a while and it annoyed me, but I upgraded at least one of the hosts since then). I am ok either way. To not wait for it and since CI is happy, will merge as is for now but will add a label for potential CP to maint.