fix: Exclude external directory when generating python report

[ewianda]

This PR will reduce the time it take to run bazel coverage

Before this fix,

bazel coverage  --cache_test_results=no  //...
Elapsed time: 7.054s, Critical Path: 6.87s

lcov --list bazel-out/_coverage/_coverage_report.dat

After

 bazel coverage  --cache_test_results=no  //...
Elapsed time: 2.474s, Critical Path: 1.18s


$ lcov --list bazel-out/_coverage/_coverage_report.dat 
Reading tracefile bazel-out/_coverage/_coverage_report.dat
                                          |Lines       |Functions  |Branches    
Filename                                  |Rate     Num|Rate    Num|Rate     Num
================================================================================
[/home/ewianda/.cache/bazel/_bazel_ewianda/da4b4cc49e0e621570c9e24d6f1eab95/execroot/_main/bazel-out/k8-fastbuild/bin/benchsci/ml/nlp/]
test_tokenizer_stage2_bootstrap.py        | 6.0%    250|    -     0|    -      0

[benchsci/]
devtools/python/pytest_helper.py          |90.5%     21|    -     0|    -      0
ml/nlp/test_tokenizer.py                  | 100%     13|    -     0|    -      0
ml/nlp/tokenizer.py                       |61.8%     76|    -     0|    -      0
================================================================================
                                    Total:|26.1%    360|    -     0|    -      0

Related to #1434

[aignas]

LGTM, could you please add a note in the Changelog for this, please?

[aignas]

Thank you very much for the really nice comment!