Implement timeout.at(when)

[asvetlov]

To cancel at the specified time (previously the delay from now was only supported).

[codecov]

Codecov Report

Merging #117 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #117   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           1      1           
  Lines          75     84    +9     
  Branches       12     14    +2     
=====================================
+ Hits           75     84    +9

Impacted Files	Coverage Δ
async_timeout/__init__.py	100% <100%> (ø)	⬆️

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