Fix setup.cfg format issue to match old setuptools versions.

[liushilongbuaa]

install_requires = file: requirements.txt

This kind of format will break old setuptools versions.
Currently requirements.txt is empty. Maybe it's better to remove this line temporarily.

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[antoninbas]

Why did you open a new PR and why are you no longer removing requirements.txt?
(it's also still asking you to sign the ONF CLA)

[antoninbas]

@liushilongbuaa please accept the ONF CLA and address review comments, so we can move forward with this
otherwise, we will need to open a replacement PR so we can fix the issue

[antoninbas]

@liushilongbuaa Thanks for bringing this to our attention and for the PR. We merged an alternative PR to this one (#199) in order to get a fix in faster, since we haven't heard from you in a couple of days.