Disallow creation of aiohttp objects without running event loop

[asvetlov]

ClientSession, Connector and others are affected.

[kxepal]

👍

[codecov-io]

Codecov Report

Merging #3539 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3539      +/-   ##
==========================================
+ Coverage   97.88%   97.92%   +0.04%     
==========================================
  Files          44       44              
  Lines        8615     8643      +28     
  Branches     1390     1404      +14     
==========================================
+ Hits         8433     8464      +31     
  Misses         75       75              
+ Partials      107      104       -3

Impacted Files	Coverage Δ
aiohttp/helpers.py	97.22% <100%> (+0.48%)	⬆️
aiohttp/__init__.py	95.83% <0%> (-4.17%)	⬇️
aiohttp/client_reqrep.py	97.84% <0%> (+0.41%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2bec332...190a277. Read the comment docs.

[blueyed]

Can you explain the reason for this?
I've thought that having it on the module level would be good / comfortable way to share it / use a single instance only.