fix _exit_event.set in __aexit__ coroutine of paginator

[mklokocka]

Proposed fix for the self._exit_event of paginator not being correctly set when task using paginator cancelled.

[codecov]

Codecov Report

Merging #40 into master will decrease coverage by 3.77%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #40      +/-   ##
==========================================
- Coverage     100%   96.22%   -3.78%     
==========================================
  Files           1        1              
  Lines         106      106              
  Branches       14       14              
==========================================
- Hits          106      102       -4     
- Misses          0        3       +3     
- Partials        0        1       +1

Impacted Files	Coverage Δ
aiocassandra.py	96.22% <100%> (-3.78%)	⬇️

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