Update cmd2 dependency for py3.8

[nzlosh]

WIP to fix CI error

   Obtaining st2sdk from git+https://github.com/StackStorm/st2sdk.git@master#egg=st2sdk (from -r /home/runner/work/stackstorm-vault/stackstorm-vault/requirements-dev.txt (line 6))
    Cloning https://github.com/StackStorm/st2sdk.git (to revision master) to /home/runner/virtualenv/src/st2sdk
      ERROR: Command errored out with exit status 1:
       command: /home/runner/virtualenv/bin/python -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/home/runner/virtualenv/src/st2sdk/setup.py'"'"'; __file__='"'"'/home/runner/virtualenv/src/st2sdk/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' egg_info --egg-base /tmp/pip-pip-egg-info-3ve5hs7d
           cwd: /home/runner/virtualenv/src/st2sdk/
      Complete output (3 lines):
      error in st2sdk setup command: 'install_requires' must be a string or list of strings containing valid project/version requirement specifiers; Expected end or semicolon (after version specifier)
          cmd2>=0.9.i5,<0.10
              ~~~~~^

[nzlosh]

Thanks, this has fixed the CI error for st2sdk.