bootstrap: Conditionally use --use-feature=2020-resolver for pip

bootstrap: Conditionally use --use-feature=2020-resolver for pip

pip might be too old so the --use-feature flag is not available. That
leads to:

$ ./bootstrap
Removing old virtualenv because it uses system site-packages
created virtual environment CPython3.8.5.final.0-64 in 173ms
  creator CPython3Posix(dest=/home/tom/devel/ceph/teuthology/virtualenv, clear=False, global=False)
  seeder FromAppData(download=False, pip=bundle, setuptools=bundle, wheel=bundle, via=copy, app_data_dir=/home/tom/.local/share/virtualenv)
    added seed packages: pip==20.1.1, setuptools==50.3.0, wheel==0.35.1
  activators BashActivator,CShellActivator,FishActivator,PowerShellActivator,PythonActivator,XonshActivator

Usage:
  pip install [options] <requirement specifier> [package-index-options] ...
  pip install [options] -r <requirements file> [package-index-options] ...
  pip install [options] [-e] <vcs project url> ...
  pip install [options] [-e] <local project path> ...
  pip install [options] <archive url/path> ...

no such option: --use-feature

So for the pip upgrade itself, check if the flag is available. From
there on, use the new resolver.

Signed-off-by: Thomas Bechtold <tbechtold@suse.com>