Version changes for v6.8.0

…e-newer

ci: Exempt plotly.py from exclude-newer rule

Closes #4654.

Passing an unsupported value to marginal_x, marginal_y or marginal in
Plotly Express (for example marginal_x="density") left trace_spec as None
in make_trace_spec, which later failed deep inside make_figure with a
confusing "'NoneType' object has no attribute 'constructor'".

make_trace_spec now raises a ValueError naming the offending value and
listing the supported marginal plot types: rug, box, violin and
histogram. The singular `marginal` argument is normalised to marginal_x
or marginal_y before this point, so the single check covers all three
parameters.

Adds a regression test in tests/test_optional/test_px/test_marginals.py.

…ssage

Raise a clear error for unsupported marginal plot types

Follow-up to #5625. The f-string refactor merged there concatenated two
adjacent string literals without a separating space, so the message
rendered as "...for `marginal_x`.Supported marginal plot types...". This
restores the space after the period.

…-spacing

Add missing space in unsupported-marginal error message

…ild artifacts must be updated when updating plotly.js version

chore: Update plotly.js to v3.7.0