Fix failing CI on main

[jsignell]

CI is failing on main with errors like:

ERROR: usage: __main__.py [options] [file_or_dir] [file_or_dir] [...]
__main__.py: error: unrecognized arguments: --mypy-only-local-stub
  inifile: /home/runner/work/xarray/xarray/pyproject.toml
  rootdir: /home/runner/work/xarray/xarray

This PR changes removes "mypy-only-local-stub" to "mypy-no-silence-site-packages" because it feels like what we are seeing is probably https://github.com/typeddjango/pytest-mypy-plugins/blob/4.0.0/CHANGELOG.md?plain=1#L7-L9

[jsignell]

@chuckwondo do you think this is the right fix?

[chuckwondo]

Good question. Based upon the changelog entry, I'm wondering if we simply want to drop the removed flag rather than replace it with the new flag. Since the new flag is intended to restore the behavior of the removed flag, I'm thinking we might not want to do that since, "Errors are now ignored in site-packages by default."

[jsignell]

You are a better reader than me! I think you are 100% right

[jsignell]

Is someone available to approve this? Maybe @keewis or @VeckoTheGecko?

[dcherian]

In general, anyone with merge rights should feel empowered to click the green button based on their personal judgement of risks involved. We all trust your judgement! And in the rare event that the PR is flawed, reverts are cheap and easy.