Add tests for python3.8 and python3.9

[mylibrar]

.github/workflows/main.yml:

• Add three tests:
  • python3.8, torch[1.7.1, 1.8.1]
  • python3.9, torch1.8.1
• Bypass mypy tests for torch[1.7.1, 1.8.1]

[codecov]

Codecov Report

Merging #340 (09a3878) into master (028c1a8) will increase coverage by 0.25%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #340      +/-   ##
==========================================
+ Coverage   80.18%   80.43%   +0.25%     
==========================================
  Files         135      135              
  Lines       11223    11357     +134     
==========================================
+ Hits         8999     9135     +136     
+ Misses       2224     2222       -2     

Impacted Files	Coverage Δ
texar/torch/version.py	100.00% <100.00%> (ø)
texar/torch/data/data/scalar_data.py	100.00% <0.00%> (ø)
texar/torch/modules/decoders/gpt2_decoder.py	100.00% <0.00%> (ø)
...exar/torch/data/tokenizers/gpt2_tokenizer_utils.py	100.00% <0.00%> (ø)
...ar/torch/modules/classifiers/roberta_classifier.py	100.00% <0.00%> (ø)
...exar/torch/modules/encoders/transformer_encoder.py	97.87% <0.00%> (+0.02%)	⬆️
texar/torch/evals/bleu_transformer.py	97.77% <0.00%> (+0.02%)	⬆️
...xar/torch/modules/decoders/transformer_decoders.py	94.52% <0.00%> (+0.02%)	⬆️
texar/torch/run/executor_utils.py	83.58% <0.00%> (+0.06%)	⬆️
texar/torch/modules/networks/conv_networks.py	86.04% <0.00%> (+0.06%)	⬆️
... and 37 more

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