style: 💄 typing improvements

[ringohoffman]

I have a draft for #113, and these are some unrelated changes I made in preparation:

1. CLIArgs argparse.Namespace dataclass for improved type checking in pybind11_stubgen/__init__.py
2. not necessary to index sys.version_info, and doing so interferes with VS Code's reachability highlighting
3. custom all_isinstance TypeGuard in pybind11_stubgen/parser/mixins/fix.py to resolve a type error in arguments to FixedSize vs DynamicSize
4. use Sequence to accept broader types
5. typo in FixMissingImport.handle_type?
6. use comprehension instead of map

[sizmailov]

Thanks!