Fixes the deprecation warning

[safiyat]

Fixes the warning:
DEPRECATION: pip install --download has been deprecated and will be removed in
the future. Pip now has a download command that should be used instead.

Uses the new pip download command instead of the deprecated pip install.

[Jay54520]

The original command is pip install -d, means -d, --download <dir> Download packages into <dir> instead of installing them, regardless of what's already installed.
You should update to pip download, the -d should be removed.

According to https://pip.pypa.io/en/stable/reference/pip_download/.

[Jay54520]

I also create a issue -- #71 for this. When the merge request is accepted, the issue should be closed. So your description should change to Fixes #71/Fixes the deprecation warning

[safiyat]

@Jay54520 The -d option is to specify the destination directory.

Please check: https://pip.pypa.io/en/stable/reference/pip_download/#cmdoption-d

[Jay54520]

@safiyat Yes, same effect between pip install -d and pip download -d, I have no problem with the pull request.

[safiyat]

Closing in favour of #75