Drop python 3.7 support

[Pliner]

It is not supported anymore.

https://endoflife.date/python

[codecov[bot]]

Codecov Report

Merging #902 (fe9916c) into master (7ae1a58) will increase coverage by 0.34%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #902      +/-   ##
==========================================
+ Coverage   93.11%   93.46%   +0.34%     
==========================================
  Files          12       12              
  Lines        1584     1576       -8     
  Branches      188      186       -2     
==========================================
- Hits         1475     1473       -2     
+ Misses         77       72       -5     
+ Partials       32       31       -1     

Files Changed	Coverage Δ
aiopg/utils.py	92.78% <100.00%> (+5.16%)	:arrow_up:

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