dependencies/boost: Fix a number of mypy issues
dependencies/boost: Fix a number of mypy issues

Since mypy can now see type information for mesonlib it can determine a
number of potential errors in the boost module, this fixes those.

QuLogic/meson,pexip/meson,mesonbuild/meson,pexip/meson,QuLogic/meson,jpakkane/meson,mesonbuild/meson,mesonbuild/meson,jpakkane/meson,pexip/meson,mesonbuild/meson,pexip/meson,QuLogic/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,pexip/meson,jpakkane/meson,becm/meson,jpakkane/meson,pexip/meson,jpakkane/meson,QuLogic/meson,QuLogic/meson,pexip/meson,pexip/meson,QuLogic/meson,QuLogic/meson,becm/meson,QuLogic/meson,QuLogic/meson,becm/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,becm/meson,becm/meson,becm/meson,becm/meson,jpakkane/meson,jpakkane/meson,jpakkane/meson,becm/meson,mesonbuild/meson,becm/meson,becm/meson,mesonbuild/meson,jpakkane/meson