tools/setup-virtualenv.sh: pass --use-feature=2020-resolver to pip

tools/setup-virtualenv.sh: pass --use-feature=2020-resolver to pip

as long as pip supports this option, pass it to `pip install`

to silence warnings and errors like:

ERROR: After October 2020 you may experience errors when installing or updating packages. This is because pip will change the way that it resolves dependency conflicts.

We recommend you use --use-feature=2020-resolver to test your packages with the new resolver before it becomes the default.

autopep8 1.5.4 requires pycodestyle>=2.6.0, but you'll have pycodestyle 2.5.0 which is incompatible.
pytest-cov 2.10.1 requires pytest>=4.6, but you'll have pytest 3.10.1 which is incompatible.

Signed-off-by: Kefu Chai <kchai@redhat.com>