Added missing slots to context managers

[Pliner]

I've added missing slots to several context managers

[codecov]

Codecov Report

Merging #763 (93d6dda) into master (78543a1) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #763      +/-   ##
==========================================
+ Coverage   92.76%   92.78%   +0.02%     
==========================================
  Files          13       13              
  Lines        1562     1567       +5     
  Branches      179      179              
==========================================
+ Hits         1449     1454       +5     
  Misses         81       81              
  Partials       32       32              

Impacted Files	Coverage Δ
aiopg/__init__.py	100.00% <100.00%> (ø)
aiopg/utils.py	80.64% <100.00%> (+0.64%)	⬆️

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