Ensure executor attributes are defined by subclasses

[quantum5]

No description provided.

[codecov-io]

Codecov Report

Merging #524 into master will increase coverage by 0.17%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #524      +/-   ##
==========================================
+ Coverage   72.78%   72.96%   +0.17%     
==========================================
  Files         126      127       +1     
  Lines        3925     3958      +33     
==========================================
+ Hits         2857     2888      +31     
- Misses       1068     1070       +2

Impacted Files	Coverage Δ
dmoj/executors/compiled_executor.py	94.78% <100%> (ø)	⬆️
dmoj/executors/mixins.py	93.33% <100%> (ø)	⬆️
dmoj/executors/metaclass.py	100% <100%> (ø)
dmoj/utils/unicode.py	77.77% <0%> (ø)	⬆️
dmoj/executors/base_executor.py	78.44% <0%> (+0.45%)	⬆️
dmoj/cptbox/tracer.py	83.16% <0%> (+0.49%)	⬆️
dmoj/checkers/sorted.py	96.55% <0%> (+1.55%)	⬆️

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