Remove in-tree-build flag from pip

[speth]

This flag doesn't work with pip versions older than 21.1, but Cantera builds fine without it.

The indicated behavior becomes default in 21.3 anyway, and then starts generating warnings of its own in pip 22.0.

If applicable, fill in the issue number this pull request is fixing

Fixes #1269

Checklist

• The pull request includes a clear description of this code change
• Commit messages have short titles and reference relevant issues
• Build passes (scons build & scons test) and unit tests address code coverage
• Style & formatting of contributed code follows contributing guidelines
• The pull request is ready for review

[bryanwweber]

Thanks @speth, this seems fine to me. That deprecation-to-removal window is very short... Just wanted to link the related issue and PR here that introduced this flag: #1260 and #1261

[ischoegl]

Is there a way to condition the flags on the pip version? If I understand the original intent of #1261, the motivation was to anticipate:

DEPRECATION: A future pip version will change local packages to be built in-place without first copying 
to a temporary directory. We recommend you use --use-feature=in-tree-build to test your packages with this 
new behavior before it becomes the default. pip 21.3 will remove support for this functionality. 
You can find discussion regarding this at https://github.com/pypa/pip/issues/7555.

One way to handle this is to be explicit with some Scons options: as @bryanwweber suggested in #1260, there is the scons build env_vars=TMPDIR option?

[speth]

Yes, that was the original intent, but there's already nothing to anticipate -- pip made this behavior the default in version 21.3 (released 2021-10-11), so we're effectively using this everywhere we have a current pip install.

I don't think it's worth introducing a special case for the very narrow window of pip versions where there is a choice (and where Cantera's build process is generally indifferent to that choice).