BLD: Install the type stub for backends.registry - #32481

PR summary

lib/matplotlib/backends/registry.pyi was added in 3.9 (#27719) but is not
listed in typing_sources in lib/matplotlib/backends/meson.build, so it
has never been installed. Wheels from 3.9.0 to 3.11.2 ship registry.py
without it.

Because py.typed is installed, type checkers read the unannotated
registry.py instead. They still know that backend_registry is a
BackendRegistry, but all its methods are untyped. With mypy on an
installed wheel:

from matplotlib.backends.registry import BackendFilter, backend_registry

reveal_type(backend_registry.list_builtin(BackendFilter.INTERACTIVE))
# before: Any    after: list[str]
reveal_type(backend_registry.resolve_gui_or_backend("qt"))
# before: Any    after: tuple[str, str | None]

Under mypy --strict, each of these calls is currently a
no-untyped-call error.

This PR adds the file to the list. A wheel built with the change has
registry.pyi as its only new file, and stubtest reports no differences
between the stub and the runtime module. I ran it without the allowlist,
because ci/mypy-stubtest-allowlist.txt skips everything under
matplotlib.backends.

The two other uninstalled stubs are left out on purpose:

• a) backends/backend_pdf.pyi has annotation errors that would reject valid
  code once installed, e.g. PdfPages.savefig(fig, dpi=300). I'll fix and
  install it in a follow-up PR.
• b) dates.pyi covers only the module's functions, so installing it would make
  from matplotlib.dates import DateFormatter a type error. It can be added
  once the stub is complete (Add type hints for matplotlib.dates module #30126).

AI Disclosure

Claude Code (Claude Opus 5.5 (high) ) flagged this while making a core audit. This change was also verified by Codex ( GPT-6.1 sol (xhigh) ).

PR quality check

• Use an expressive title, e.g. "Fix title font property precedence"
• [N/A] New and changed code is tested (install list only; checked by building a wheel)
• [N/A] Plotting related features are demonstrated in an example
• [N/A] New features and API changes have release notes
• [N/A] Documentation complies with general and docstring guidelines