arglist: Fix remaining mypy errors and warnings

So we can lint it with mypy

.github/workflows/lint_mypy.yml

@@ -31,4 +31,4 @@ jobs:
       with:
         python-version: '3.x'
     - run: python -m pip install mypy
-    - run: mypy --follow-imports=skip mesonbuild/interpreterbase.py mesonbuild/mtest.py mesonbuild/minit.py mesonbuild/mintro.py mesonbuild/mparser.py mesonbuild/msetup.py mesonbuild/ast mesonbuild/wrap tools/ mesonbuild/modules/fs.py mesonbuild/dependencies/boost.py mesonbuild/dependencies/mpi.py mesonbuild/dependencies/hdf5.py mesonbuild/compilers/mixins/intel.py mesonbuild/mlog.py mesonbuild/mcompile.py mesonbuild/mesonlib.py
+    - run: mypy --follow-imports=skip mesonbuild/interpreterbase.py mesonbuild/mtest.py mesonbuild/minit.py mesonbuild/mintro.py mesonbuild/mparser.py mesonbuild/msetup.py mesonbuild/ast mesonbuild/wrap tools/ mesonbuild/modules/fs.py mesonbuild/dependencies/boost.py mesonbuild/dependencies/mpi.py mesonbuild/dependencies/hdf5.py mesonbuild/compilers/mixins/intel.py mesonbuild/mlog.py mesonbuild/mcompile.py mesonbuild/mesonlib.py mesonbuild/arglist.py

mesonbuild/arglist.py

@@ -26,7 +26,6 @@ if T.TYPE_CHECKING:
     from .linkers import StaticLinker
     from .compilers import Compiler
 
-
 UNIXY_COMPILER_INTERNAL_LIBS = ['m', 'c', 'pthread', 'dl', 'rt']  # type: T.List[str]
 # execinfo is a compiler lib on FreeBSD and NetBSD
 if mesonlib.is_freebsd() or mesonlib.is_netbsd():
@@ -159,7 +158,7 @@ class CompilerArgs(collections.abc.MutableSequence):
         pass
 
     @T.overload                                                     # noqa: F811
-    def __getitem__(self, index: slice) -> T.List[str]:  # noqa: F811
+    def __getitem__(self, index: slice) -> T.MutableSequence[str]:  # noqa: F811
         pass
 
     def __getitem__(self, index):  # noqa: F811
@@ -171,7 +170,7 @@ class CompilerArgs(collections.abc.MutableSequence):
         pass
 
     @T.overload                                                       # noqa: F811
-    def __setitem__(self, index: slice, value: T.List[str]) -> None:  # noqa: F811
+    def __setitem__(self, index: slice, value: T.Iterable[str]) -> None:  # noqa: F811
         pass
 
     def __setitem__(self, index, value) -> None:  # noqa: F811
@@ -312,7 +311,7 @@ class CompilerArgs(collections.abc.MutableSequence):
         new += self
         return new
 
-    def __eq__(self, other: T.Any) -> T.Union[bool, type(NotImplemented)]:
+    def __eq__(self, other: T.Any) -> T.Union[bool]:
         self.flush_pre_post()
         # Only allow equality checks against other CompilerArgs and lists instances
         if isinstance(other, CompilerArgs):