Support pip's --user option in mypy's --install-types

[lsfischer]

Fixes: #16339

This PR adds support for a new command line argument --user. This command line argument is intended to be used together with --install-types throwing an error otherwise.
When used with --install-types, this new option will pass the --user flag to pip install so that the missing library stubs are installed to the user's local python package install location.

Example

$ mypy --install-types --non-interactive --user -c "import lxml"
Installing missing stub packages:
/home/lsfischer/anaconda3/envs/mypy/bin/python -m pip install --user lxml-stubs

Collecting lxml-stubs
  Downloading lxml_stubs-0.4.0-py3-none-any.whl (13 kB)
Installing collected packages: lxml-stubs
Successfully installed lxml-stubs-0.4.0

[lsfischer]

If more pip options are supported in the future, this could go into a proper derive_pip_options function or similar

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[lsfischer]

Pinging maybe @AlexWaygood, I would love to get your feedback on this topic.
I feel this would be good alternative to people without a venv

[gtedesco-r7]

For anyone else searching for this export PIP_USER=true or setting a pip.conf can achieve the same thing for now.

[hauntsaninja]

Thanks for the PR! I'm not planning on merging this. I don't want --install-types to replicate pip's user interface. As it is it has some usability issues, e.g. it effectively means you have to run mypy twice. It sounds like there are reasonable workarounds mentioned by gtedesco-r7