This will resolve the bug in BF-26744.
There have been reports of the following error when using --system-site-packages in a virtual environment.
AttributeError: module 'setuptools.build_meta' has no attribute '__legacy__'
https://github.com/pypa/setuptools/issues/1694
https://github.com/pypa/pip/issues/6264
On other branches (5.0, 6.0, master), we don't use --system-site-packages.
We should remove --system-site-packages from virtualenv setup on 4.4 since it doesn't provide much value in our case.