Make strict_optional opt out for mesonbuild, not opt in

[bonzini]

I'm not too interested in getting this forward, but I can at least try to avoid making it worse.

Fix some of the lowest hanging fruit and list all failing modules in .mypy.ini.

Here are the remaining files with 10 or less errors (80% of the files, 25% of the errors):

mesonbuild/scripts/coverage.py 10
mesonbuild/ast/introspection.py 10
mesonbuild/interpreter/dependencyfallbacks.py 9
mesonbuild/dependencies/python.py 9
mesonbuild/dependencies/configtool.py 9
mesonbuild/msetup.py 8
mesonbuild/modules/python.py 8
mesonbuild/modules/__init__.py 8
mesonbuild/interpreterbase/decorators.py 8
mesonbuild/dependencies/dev.py 8
mesonbuild/compilers/mixins/clike.py 8
mesonbuild/utils/universal.py 7
mesonbuild/dependencies/misc.py 7
mesonbuild/compilers/rust.py 7
mesonbuild/compilers/mixins/visualstudio.py 7
mesonbuild/cmake/interpreter.py 7
mesonbuild/linkers/detect.py 6
mesonbuild/dependencies/qt.py 6
mesonbuild/wrap/wrap.py 5
mesonbuild/scripts/scanbuild.py 5
mesonbuild/msubprojects.py 5
mesonbuild/modules/pkgconfig.py 5
mesonbuild/compilers/d.py 5
mesonbuild/options.py 4
mesonbuild/modules/cmake.py 4
mesonbuild/mintro.py 4
mesonbuild/dependencies/boost.py 4
mesonbuild/compilers/detect.py 4
mesonbuild/cmake/toolchain.py 4
mesonbuild/modules/_qt.py 3
mesonbuild/modules/gnome.py 3
mesonbuild/interpreter/interpreterobjects.py 3
mesonbuild/dependencies/pkgconfig.py 3
mesonbuild/dependencies/detect.py 3
mesonbuild/dependencies/base.py 3
mesonbuild/compilers/mixins/islinker.py 3
mesonbuild/cmake/tracetargets.py 3
mesonbuild/wrap/wraptool.py 2
mesonbuild/modules/cuda.py 2
mesonbuild/modules/codegen.py 2
mesonbuild/dependencies/ui.py 2
mesonbuild/dependencies/framework.py 2
mesonbuild/compilers/mixins/clang.py 2
mesonbuild/compilers/mixins/apple.py 2
mesonbuild/mparser.py 1
mesonbuild/interpreterbase/baseobjects.py 1
mesonbuild/dependencies/mpi.py 1
mesonbuild/dependencies/hdf5.py 1
mesonbuild/dependencies/dub.py 1
mesonbuild/compilers/vala.py 1
mesonbuild/compilers/mixins/elbrus.py 1
mesonbuild/compilers/mixins/arm.py 1
mesonbuild/compilers/cython.py 1
mesonbuild/compilers/cuda.py 1

The worst offenders (10% of the files, 50% of the issues) are

mesonbuild/backend/ninjabackend.py 150
mesonbuild/backend/backends.py 118
mesonbuild/mformat.py 90
mesonbuild/backend/vs2010backend.py 68
mesonbuild/scripts/depfixer.py 48
mesonbuild/backend/xcodebackend.py 43

and it's likely that something clever will fix most of the issues.