cleanup: remove core-v3 because core support python2/3 both

[JacksonTian]

You need to complete

• [x] unit tests and/or feature tests
• [ ] documentation is changed or added

[codecov[bot]]

Codecov Report

Merging #509 (3fc55b0) into master (894ebbb) will not change coverage. The diff coverage is n/a.

:exclamation: Current head 3fc55b0 differs from pull request most recent head 3248c81. Consider uploading reports for the commit 3248c81 to get more accurate results

@@           Coverage Diff           @@
##           master     #509   +/-   ##
=======================================
  Coverage   95.92%   95.92%           
=======================================
  Files          45       45           
  Lines        1916     1916           
  Branches      276      276           
=======================================
  Hits         1838     1838           
  Misses         56       56           
  Partials       22       22