Make pdb.orig_set_trace available

[blueyed]

Seems to be necessary for PYTHONBREAKPOINT.

PYTHONBREAKPOINT=pdb.pdb.set_trace python -c 'breakpoint()'
-c:1: RuntimeWarning: Ignoring unimportable $PYTHONBREAKPOINT: "pdb.pdb.set_trace"

I have improved this for pytest's --pdbcls to also handle attributes, but am not sure if it is acceptable/worth it for Python itself..
Ref: pytest-dev/pytest#4855

Moved from pdbpp/pdbpp#142.

[codecov-io]

Codecov Report

Merging #4 into master will decrease coverage by 0.12%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master       #4      +/-   ##
==========================================
- Coverage    66.7%   66.57%   -0.13%     
==========================================
  Files           3        3              
  Lines        2580     2585       +5     
  Branches      228      228              
==========================================
  Hits         1721     1721              
- Misses        784      790       +6     
+ Partials       75       74       -1

Impacted Files	Coverage Δ
testing/test_pdb.py	81.01% <100%> (+0.11%)	⬆️
pdb.py	45.25% <100%> (-0.53%)	⬇️

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[blueyed]

Back to pdbpp/pdbpp#142.