py: Fix TracebackDump not to dump duplicated exception type

[corona10]

Fixes: #23

[codecov-io]

Codecov Report

Merging #36 into master will increase coverage by 0.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master      #36      +/-   ##
==========================================
+ Coverage   65.23%   65.25%   +0.01%     
==========================================
  Files          56       56              
  Lines       10151    10148       -3     
==========================================
  Hits         6622     6622              
+ Misses       3067     3064       -3     
  Partials      462      462

Impacted Files	Coverage Δ
py/exception.go	34.54% <0%> (+0.91%)	⬆️

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[corona10]

@ncw
PTAL, proper unit test can't add this time.
Maybe we can add capturing fmt.Fprintf but not the easy job to this.

[ncw]

PTAL, proper unit test can't add this time.
Maybe we can add capturing fmt.Fprintf but not the easy job to this.

Well I think it is a definite improvement, so I'll merge it - thank you :-)