[DOC] Remove Py2 icon from Readme. Add 3.7

[leezu]

Description

Remove Py2 icon from Readme. Add 3.7

Comments

Our CI does not test 3.7. Code may be broken on 3.7, though it's unlikely. I use 3.7 locally and haven't experienced problems.

[codecov]

Codecov Report

Merging #856 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #856   +/-   ##
=======================================
  Coverage   90.18%   90.18%           
=======================================
  Files          66       66           
  Lines        6345     6345           
=======================================
  Hits         5722     5722           
  Misses        623      623

[mli]

Found link check problems in job PR-856/3:
(line 19) broken https://www.salesforce.com/products/einstein/ai-research/the-wikitext-dependency-language-modeling-dataset/ - 404 Client Error: Not Found for url: https://www.salesforce.com/products/einstein/ai-research/the-wikitext-dependency-language-modeling-dataset/
(line 5) broken https://www.salesforce.com/products/einstein/ai-research/the-wikitext-dependency-language-modeling-dataset/ - 404 Client Error: Not Found for url: https://www.salesforce.com/products/einstein/ai-research/the-wikitext-dependency-language-modeling-dataset/
(line 5) broken https://www.salesforce.com/products/einstein/ai-research/the-wikitext-dependency-language-modeling-dataset/ - 404 Client Error: Not Found for url: https://www.salesforce.com/products/einstein/ai-research/the-wikitext-dependency-language-modeling-dataset/
(line 21) broken https://www.salesforce.com/products/einstein/ai-research/the-wikitext-dependency-language-modeling-dataset/ - 404 Client Error: Not Found for url: https://www.salesforce.com/products/einstein/ai-research/the-wikitext-dependency-language-modeling-dataset/
(line 442) broken https://www.aclweb.org/anthology/P02-1040.pdf)[1 - 404 Client Error: Not Found for url: https://www.aclweb.org/anthology/P02-1040.pdf)%5B1
(line 92) broken https://nlp.stanford.edu/pubs/glove.pdf)[2 - 404 Client Error: NOT FOUND for url: https://nlp.stanford.edu/pubs/glove.pdf)%5B2
(line 208) broken https://www.bioinf.jku.at/publications/older/2604.pdf)[3 - 404 Client Error: Not Found for url: https://www.bioinf.jku.at/publications/older/2604.pdf)%5B3

[mli]

Job PR-856/3 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-856/3/index.html