pip warning on CI

[dcherian]

We're getting this warning on CI (e.g. https://dev.azure.com/xarray/xarray/_build/results?buildId=2047&view=logs&j=e9c23135-6f4c-5980-91c2-81d28ce70c9b&t=bd167cdf-93c0-5e40-5b11-b5faeb5dc22f)

WARNING: pip is being invoked by an old script wrapper. This will fail in a future version of pip.
Please see https://github.com/pypa/pip/issues/5599 for advice on fixing the underlying issue.
To avoid this problem you can invoke Python with '-m pip' instead of running pip directly.

cc @crusaderky