Fix race condition in dispatcher start/stop

[tsnoam]

Fixes #881

[jsmnbom]

LGTM, seems like a simple fix.
Can we test it?

[jh0ker]

Shouldn't we do ready.set() here as well?

[tsnoam]

that makes sense. i'll fix.

[codecov]

Codecov Report

Merging #887 into master will decrease coverage by 0.18%.
The diff coverage is 75%.

@@            Coverage Diff            @@
##           master    #887      +/-   ##
=========================================
- Coverage   91.89%   91.7%   -0.19%     
=========================================
  Files         101     101              
  Lines        4057    4063       +6     
  Branches      621     623       +2     
=========================================
- Hits         3728    3726       -2     
- Misses        194     198       +4     
- Partials      135     139       +4

Flag	Coverage Δ
#Appveyor	86.85% <75%> (-0.03%)	⬇️
#Travis	91.28% <75%> (-0.19%)	⬇️

Impacted Files	Coverage Δ
telegram/ext/updater.py	76.92% <100%> (+0.25%)	⬆️
telegram/ext/dispatcher.py	91.81% <60%> (-1.01%)	⬇️
telegram/message.py	96.22% <0%> (-0.76%)	⬇️
telegram/bot.py	87.22% <0%> (-0.52%)	⬇️

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[jh0ker]

Love it :)

[tsnoam]

@bomjacob I don't think that we can properly test it.
We can pass an Event object to dispatcher.start() but what will it tell us? Only that we've called set() on the Event. It won't guarantee that it had been done properly.
So basically, I don't think that testing this is worth the effort.

Edit: properly = in proper order