Following "alexandria should just work" principle: PDF URLs (arxiv,
whitepapers) are a first-class capability, not something users should
have to opt into with an extra. Promoting pymupdf from the [pdf] extra
to a core dependency. The [pdf] extra is retained as a no-op alias for
back-compat so existing `pip install 'alexandria-wiki[pdf]'` commands
don't break.

* pyproject: pymupdf moved from [project.optional-dependencies].pdf to
  [project].dependencies; [pdf] extra left as []
* alexandria/core/{web.py,pdf.py}: drop the defensive ImportError
  handling and misleading install hint — pymupdf is always present now
* README: drop the [pdf] install line; core install covers PDF
* Bump version to 0.37.1