unpin `pydata-sphinx-theme==0.15.2`

[stevepiercy]

When executablebooks/sphinx-book-theme#865 is resolved, repin dependencies to latest in pyproject.toml.

dependencies = [
  # stable pins, see https://github.com/plone/plone-sphinx-theme/issues/42
  "myst-parser==3.0.1",
  "pydata-sphinx-theme==0.15.4",
  "sphinx==7.3.7",
  "sphinx-book-theme==1.1.3",
]

Upstream of Sphinx Book Theme, this is the issue to watch: pydata/pydata-sphinx-theme#2067

[stevepiercy]

I updated some dependencies in #56.