chore: require Python 3.8 on 3.x, since ipyvue 3.1.0 does not import on 3.7

[maartenbreddels]

ipyvuetify 3.x now needs Python 3.8 or newer, because its dependency ipyvue 3.1.0 no longer imports on Python 3.7.

Problem

ipyvuetify 3.x depends on ipyvue>=3.0.0. ipyvue 3.1.0 uses an assignment expression (:=), which Python 3.7 cannot parse.
So on Python 3.7, pip installs ipyvuetify and ipyvue 3.1.0, and then import ipyvuetify fails with a SyntaxError.
This repository's test (3.7) CI job now fails for that reason, also on unrelated PRs such as #346.
Python 3.7 has been end of life since June 2023.

Change

• pyproject.toml sets requires-python = ">=3.8" and drops the Python 3.6 and 3.7 classifiers.
• The test matrix in .github/workflows/test.yml drops Python 3.7.
• The 1.x line does not change.

Validation

• The CI run of perf: let nodeps.js use the host's Vuetify instead of a second copy #346 against ipyvue 3.1.0 passed every job except test (3.7), which failed in from ipyvue import VueWidget with SyntaxError at ipyvue/esm.py:91.
• CI on this PR runs the remaining Python versions.

Gaps

• ipyvue gets the same change in chore: require Python 3.8, since ipyvue 3.1.0 does not import on 3.7 ipyvue#117. Until a new ipyvue release carries it, and 3.1.0 is yanked, Python 3.7 users who install ipyvuetify still get a broken ipyvue 3.1.0.

Align results

• Drop Python 3.7: Maarten, 2026-10-05: "lets drop 3.7 right? for 3.x".

Crossreview results

Round 1, by astra and gpt-6.1-sol, on both companion PRs together. Neither found a defect. Both noted older things this PR does not change: some ipyvue UI-test skip messages still name old Python versions, and the setuptools minimum in ipyvuetify is out of date.

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