GitHub Template: Tag @stefan-it for token classification related bug reports

[stefan-it]

Hi,

this PR adds myself as person to tag for all token classification related bug reports :)

[codecov]

Codecov Report

Merging #6489 into master will decrease coverage by 1.47%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #6489      +/-   ##
==========================================
- Coverage   80.59%   79.11%   -1.48%     
==========================================
  Files         156      156              
  Lines       28058    28058              
==========================================
- Hits        22612    22198     -414     
- Misses       5446     5860     +414     

Impacted Files	Coverage Δ
src/transformers/modeling_tf_electra.py	25.13% <0.00%> (-73.83%)	⬇️
src/transformers/modeling_tf_openai.py	22.58% <0.00%> (-72.26%)	⬇️
src/transformers/tokenization_xlm_roberta.py	48.80% <0.00%> (-46.43%)	⬇️
src/transformers/tokenization_mbart.py	56.25% <0.00%> (-39.07%)	⬇️
src/transformers/tokenization_ctrl.py	78.64% <0.00%> (-17.48%)	⬇️
src/transformers/tokenization_transfo_xl.py	33.56% <0.00%> (-8.93%)	⬇️
src/transformers/tokenization_utils_base.py	90.24% <0.00%> (-3.53%)	⬇️
src/transformers/modeling_tf_utils.py	85.99% <0.00%> (-1.31%)	⬇️
src/transformers/modeling_openai.py	80.96% <0.00%> (-1.30%)	⬇️
src/transformers/generation_tf_utils.py	86.21% <0.00%> (-0.26%)	⬇️
... and 5 more

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[stefan-it]

@julien-c 🤔