refactor: use UPPER_CASE for Enum members

[redeboer]

Pylint 2.7.0 now checks whether members of an Enum are UPPER_CASE
http://pylint.pycqa.org/en/latest/whatsnew/changelog.html#what-s-new-in-pylint-2-7-0

Automated changes by create-pull-request GitHub action

[codecov]

Codecov Report

Merging #499 (c360ec2) into master (2061d31) will not change coverage.
The diff coverage is 92.85%.

@@           Coverage Diff           @@
##           master     #499   +/-   ##
=======================================
  Coverage   89.27%   89.27%           
=======================================
  Files          22       22           
  Lines        3338     3338           
  Branches      845      845           
=======================================
  Hits         2980     2980           
  Misses        194      194           
  Partials      164      164           

Flag	Coverage Δ
unittests	89.27% <92.85%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/expertsystem/reaction/_system_control.py	89.72% <0.00%> (ø)
src/expertsystem/amplitude/model.py	93.80% <100.00%> (ø)
src/expertsystem/reaction/__init__.py	83.57% <100.00%> (ø)
src/expertsystem/reaction/default_settings.py	80.64% <100.00%> (ø)