Add python 3.8 requirement for Whisper & update tests

[slSeanWU]

No description provided.

[sw005320]

THX!
#4886

[codecov]

Codecov Report

Merging #4891 (40080c6) into master (0f1ac0e) will decrease coverage by 0.03%.
The diff coverage is 23.52%.

@@            Coverage Diff             @@
##           master    #4891      +/-   ##
==========================================
- Coverage   76.61%   76.58%   -0.03%     
==========================================
  Files         603      603              
  Lines       53677    53707      +30     
==========================================
+ Hits        41122    41131       +9     
- Misses      12555    12576      +21     

Flag	Coverage Δ
test_integration_espnet1	66.38% <ø> (ø)
test_integration_espnet2	47.63% <17.64%> (-0.03%)	⬇️
test_python	66.45% <17.64%> (-0.03%)	⬇️
test_utils	23.35% <ø> (ø)

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Impacted Files	Coverage Δ
espnet2/asr/espnet_model.py	77.69% <7.69%> (-3.36%)	⬇️
espnet2/train/preprocessor.py	27.14% <17.64%> (-0.26%)	⬇️
espnet2/tasks/asr.py	90.72% <100.00%> (+0.04%)	⬆️
espnet2/text/cleaner.py	72.72% <100.00%> (+4.54%)	⬆️
espnet/nets/batch_beam_search_online_sim.py	92.80% <0.00%> (+1.43%)	⬆️

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