Fix for displaying help for the build_python command

[rakhmaevao]

I wanted to run the command in bar folder

jupyter-releaser build-python --help

This line is in the README.md, so it should work right away.

But it gave an error

  File "/home/rakhmaevao/Projects/jupyter_releaser/jupyter_releaser/cli.py", line 44, in invoke
    raise ValueError(msg)
ValueError: Please run prep-git first

I fixed it.

Also the command to run tests from the CONTRIBUTING.md file did not work. But another command works. I fixed the CONTRIBUTING.md file.

[rakhmaevao]

I can't put a tag. I understand that this is the problem.

[krassowski]

Thank you @rakhmaevao!