failed CI test

[cpprhtn]

CI test fails as follows
https://app.circleci.com/pipelines/github/pytorch/hub/2245/workflows/d5a70289-1cd2-4e9f-8ab8-061bacad7edf/jobs/2694

logs

Checking pytorch_vision_googlenet.md...
Traceback (most recent call last):
  File "scripts/sanity_check.py", line 160, in <module>
    sanity_check()
  File "scripts/sanity_check.py", line 150, in sanity_check
    ValidMD(f).check_markdown_file()
  File "scripts/sanity_check.py", line 143, in check_markdown_file
    self.validate_markdown(markdown)
  File "scripts/sanity_check.py", line 100, in validate_markdown
    m = mistune.create_markdown(renderer=mistune.AstRenderer())
AttributeError: module 'mistune' has no attribute 'AstRenderer'

Exited with code exit status 1
CircleCI received exit code 1

According to the history, if you fix the version of mistune before a certain point, it will be built safely.

https://pypi.org/project/mistune/#history

I'will try switching to use 2.x.x instead of the 3.x.x version.