Drop support for Python prior to 3.6

[cottsay]

Most notably, switch to f-strings.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 66.66667% with 3 lines in your changes missing coverage. Please review.

Project coverage is 44.35%. Comparing base (60aa79c) to head (44ebc89).

Files with missing lines	Patch %	Lines
colcon_cmake/task/cmake/test.py	0.00%	2 Missing ⚠️
colcon_cmake/task/cmake/build.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #149      +/-   ##
==========================================
+ Coverage   44.17%   44.35%   +0.18%     
==========================================
  Files          10       10              
  Lines         790      780      -10     
  Branches      175      172       -3     
==========================================
- Hits          349      346       -3     
+ Misses        395      389       -6     
+ Partials       46       45       -1     

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