add "w:tar" as a mode for tarfile

[asottile]

this is an alias for "w:" according to the code (which defaults the second segment to "tar")

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

vision (https://github.com/pytorch/vision)
- torchvision/datasets/utils.py:215: note:     def open(name: str | bytes | PathLike[str] | PathLike[bytes] | None, mode: Literal['x', 'x:', 'a', 'a:', 'w', 'w:'], fileobj: _Fileobj | None = ..., bufsize: int = ..., *, format: int | None = ..., tarinfo: type[TarInfo] | None = ..., dereference: bool | None = ..., ignore_zeros: bool | None = ..., encoding: str | None = ..., errors: str = ..., pax_headers: Mapping[str, str] | None = ..., debug: int | None = ..., errorlevel: int | None = ...) -> TarFile
+ torchvision/datasets/utils.py:215: note:     def open(name: str | bytes | PathLike[str] | PathLike[bytes] | None, mode: Literal['x', 'x:', 'a', 'a:', 'w', 'w:', 'w:tar'], fileobj: _Fileobj | None = ..., bufsize: int = ..., *, format: int | None = ..., tarinfo: type[TarInfo] | None = ..., dereference: bool | None = ..., ignore_zeros: bool | None = ..., encoding: str | None = ..., errors: str = ..., pax_headers: Mapping[str, str] | None = ..., debug: int | None = ..., errorlevel: int | None = ...) -> TarFile
- torchvision/datasets/utils.py:215: note:     def open(name: str | bytes | PathLike[str] | PathLike[bytes] | None = ..., *, mode: Literal['x', 'x:', 'a', 'a:', 'w', 'w:'], fileobj: _Fileobj | None = ..., bufsize: int = ..., format: int | None = ..., tarinfo: type[TarInfo] | None = ..., dereference: bool | None = ..., ignore_zeros: bool | None = ..., encoding: str | None = ..., errors: str = ..., pax_headers: Mapping[str, str] | None = ..., debug: int | None = ..., errorlevel: int | None = ...) -> TarFile
+ torchvision/datasets/utils.py:215: note:     def open(name: str | bytes | PathLike[str] | PathLike[bytes] | None = ..., *, mode: Literal['x', 'x:', 'a', 'a:', 'w', 'w:', 'w:tar'], fileobj: _Fileobj | None = ..., bufsize: int = ..., format: int | None = ..., tarinfo: type[TarInfo] | None = ..., dereference: bool | None = ..., ignore_zeros: bool | None = ..., encoding: str | None = ..., errors: str = ..., pax_headers: Mapping[str, str] | None = ..., debug: int | None = ..., errorlevel: int | None = ...) -> TarFile

schemathesis (https://github.com/schemathesis/schemathesis)
-   File "/tmp/mypy_primer/old_mypy/venv/bin/mypy", line 8, in <module>
+   File "/tmp/mypy_primer/new_mypy/venv/bin/mypy", line 8, in <module>
-   File "/tmp/mypy_primer/old_mypy/venv/lib/python3.13/site-packages/mypy/__main__.py", line 15, in console_entry
+   File "/tmp/mypy_primer/new_mypy/venv/lib/python3.13/site-packages/mypy/__main__.py", line 15, in console_entry