[refactor] Fix abstract class definitions

[vkarak]

Abstract classes must derive from abc.ABC.

[vkarak]

@jenkins-cscs retry dom

[vkarak]

@jenkins-cscs retry none

[codecov-io]

Codecov Report

Merging #805 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #805      +/-   ##
==========================================
- Coverage   91.55%   91.55%   -0.01%     
==========================================
  Files          80       80              
  Lines        9976     9975       -1     
==========================================
- Hits         9134     9133       -1     
  Misses        842      842

Impacted Files	Coverage Δ
unittests/test_modules.py	82.11% <100%> (ø)	⬆️
reframe/frontend/executors/__init__.py	98.23% <100%> (-0.01%)	⬇️
unittests/test_buildsystems.py	99.15% <100%> (ø)	⬆️
unittests/test_schedulers.py	97.67% <100%> (ø)	⬆️
reframe/core/buildsystems.py	97.11% <100%> (ø)	⬆️
reframe/utility/versioning.py	89.02% <100%> (ø)	⬆️

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