Update ci

[calumy]

Previously even if some workflow steps were failing (e.g. black and pytest), the workflow would not fail. This is not ideal as errors might not be found, and duplicate workflow runs are required to identify issues (currently Github actions and Travis).

This PR:

• Updates black and pytest steps to fail the workflow if the step fails
• Updates black step to match the configuration in the make file
• Updates flake8 to match black config

I don't have much experience with Travis CI, so reverted the attempts to update the minimum python version and black version as I could not get it to pass.

[codecov[bot]]

Codecov Report

Merging #119 (b80ae95) into master (3bba9cd) will increase coverage by 1.24%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #119      +/-   ##
==========================================
+ Coverage   94.04%   95.29%   +1.24%     
==========================================
  Files           3        3              
  Lines         168      170       +2     
  Branches       35       45      +10     
==========================================
+ Hits          158      162       +4     
+ Misses          5        4       -1     
+ Partials        5        4       -1     

Impacted Files	Coverage Δ
flake8_return/errors.py	100.00% <100.00%> (ø)
flake8_return/visitors.py	94.70% <100.00%> (+1.41%)	:arrow_up:

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[afonasev]

Thx for your contribution!)