Avoid missing-import errors for t-strings below Python 3.14

[jw9829]

Fixes #22184. Only add the implicit string.templatelib dependency when targeting Python 3.14 or newer.

[github-actions]

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

[chrikrah]

@jw9829, this is ready to merge as it stands. The version gate on compute_dependencies is the change #22184 asked for, and I falsified it at head 1d0f834 against base 0ff0c12: with mypy/build.py reverted to base and the new case kept, the case fails with the line-1 import-not-found the issue reports.

non-blocking: a .mypy_cache written by the unfixed build keeps reporting both errors under this head only where version_id is unchanged, such as the .git-less exports below or a run under --skip-version-check. Validation accepts the cached metadata before compute_dependencies runs, and build.py:2048 abandons it on a version change. A cold cache gives one syntax error:

$ for d in head falsify base; do [ -e /scratch/p18-mypy-22188/$d/.git ] && echo "$d has .git" || echo "$d no .git"; done
head no .git
falsify no .git
base no .git
$ cd /scratch/p18-mypy-22188/cachecheck && rm -rf cache
$ PYTHONPATH=../falsify /scratch/p18-mypy-22188/venv314/bin/python -m mypy --python-version 3.13 --cache-dir=cache t1.py
t1.py:1: error: Cannot find implementation or library stub for module named "string.templatelib"  [import-not-found]
t1.py:1: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
t1.py:2: error: T-strings: requires Python 3.14 or newer (current target: Python 3.13)  [syntax]
Found 2 errors in 1 file (checked 1 source file)
$ PYTHONPATH=../head /scratch/p18-mypy-22188/venv314/bin/python -m mypy --python-version 3.13 --cache-dir=cache t1.py
t1.py:1: error: Cannot find implementation or library stub for module named "string.templatelib"  [import-not-found]
t1.py:1: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
t1.py:2: error: T-strings: requires Python 3.14 or newer (current target: Python 3.13)  [syntax]
Found 2 errors in 1 file (checked 1 source file)
$ rm -rf cache && PYTHONPATH=../head /scratch/p18-mypy-22188/venv314/bin/python -m mypy --python-version 3.13 --cache-dir=cache t1.py
t1.py:2: error: T-strings: requires Python 3.14 or newer (current target: Python 3.13)  [syntax]
Found 1 error in 1 file (checked 1 source file)

t1.py is the two-line file from #22184, ../falsify is this head with mypy/build.py reverted, ../head the pull request unmodified. The suite on Python 3.14.7, then the reverted file against the new case:

$ cd /scratch/p18-mypy-22188/head && PYTHONPATH=$PWD ../venv314/bin/python -m pytest -q -n 4 mypy/test/testcheck.py
8310 passed, 27 skipped, 7 xfailed in 72.20s (0:01:12)
$ cd ../base && PYTHONPATH=$PWD ../venv314/bin/python -m pytest -q -n 4 mypy/test/testcheck.py
8309 passed, 27 skipped, 7 xfailed in 72.47s (0:01:12)
$ cd ../falsify && PYTHONPATH=$PWD ../venv314/bin/python -m pytest -q mypy/test/testcheck.py -k TemplateStringBelowTarget
FAILED mypy/test/testcheck.py::TypeCheckSuite::check-python314.test::testTemplateStringBelowTargetDoesNotImportTemplatelib
  A: main:1: error: Cannot find implementation or library stub for module named "string.templatelib"
1 failed in 1.59s
$ cd ../head && ../venv314/bin/black --check mypy/build.py
All done! 1 file would be left unchanged.
$ ../venv314/bin/ruff check mypy/build.py
All checks passed!
# not run: mypy_primer, the mypyc-compiled jobs, dmypy; black 26.10.1 and ruff 0.17.0 locally

The dependency has one writer, mypy/build.py:3447, and both existing 3.14 cases in check-python314.test still pass, so the supported path is untouched. @ilevkivskyi, you merged 9 recent changes touching these two files. version.py:15-20 ties version_id to the git revision, so merging and a release both clear that window. I'd leave the warm-cache window out of the release notes. Do you see a case I'm missing?