Made _UndefinedType as a Singleton class.

[alexhook]

PR checklist

• I've written good commit message for all commits
• I've split changes into separate commits where it's appropriate
• I've added the description of function to documentation
• I've updated project version in pyproject.toml
• I'll make a release when PR is merged
• I'll bump pybotx in bot-template

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (dc234fb) 100.00% compared to head (b928947) 100.00%.

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Additional details and impacted files

@@            Coverage Diff            @@
##            master      #402   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          107       107           
  Lines         3673      3678    +5     
  Branches       321       322    +1     
=========================================
+ Hits          3673      3678    +5     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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

Impacted Files	Coverage Δ
pybotx/missing.py	100.00% <100.00%> (ø)

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