build: use typing.Mapping for lru_cached dicts
build: use typing.Mapping for lru_cached dicts

Thankfully the typing module provides us an immutable protocol for
mappings, so we don't have to write one ourselves.

jpakkane/meson,mesonbuild/meson,jpakkane/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,jpakkane/meson,pexip/meson,pexip/meson,pexip/meson,jpakkane/meson,pexip/meson,jpakkane/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,jpakkane/meson,pexip/meson,jpakkane/meson,pexip/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,jpakkane/meson,mesonbuild/meson,jpakkane/meson