Remove dependency on the extras library

[kajinamit]

Replace the extras library by built-in items to reduce external dependency.

Note that extras.safe_hasattr was already removed[1] and should be removed.

[1] testing-cabal/extras@304c046

[mtreinish]

The code changes LGTM. Before I merge though I'll try to get the coverage ci job functional again.

[kajinamit]

@mtreinish I wonder if I can help something to move this forward, since this has been kept for some time ?

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 61.65%. Comparing base (cb0a935) to head (aeebcff).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
stestr/testlist.py	50.00%	4 Missing ⚠️
stestr/subunit_runner/program.py	33.33%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #366      +/-   ##
==========================================
- Coverage   61.76%   61.65%   -0.11%     
==========================================
  Files          30       30              
  Lines        2605     2608       +3     
  Branches      402      402              
==========================================
- Hits         1609     1608       -1     
- Misses        877      881       +4     
  Partials      119      119              

Flag	Coverage Δ
unittests	61.65% <50.00%> (-0.11%)	⬇️

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