tweak savefile_open type hints

This was fixed up in https://github.com/qutebrowser/qutebrowser/pull/8006
after a mypy update caused us to examine the typing.AnyStr thing a bit more.
But both overloads got set to have a `str` return type, so the possible
bytes return type got lost. Mypy didn't pick that up because `binary=True` is
only used in `qutebrowser/browser/webkit/cookies.py` which probably isn't
being type checked.

I had to remove the default from the binary arg of the bytes version (the ` =
...` bit) because if both overloads have the kwarg as optional mypy doesn't
know which to match a call with just one positional argument against. Eg
`savefile_open('some_file')` would match both.

I checked with reveal_type:

    diff --git i/qutebrowser/utils/qtutils.py w/qutebrowser/utils/qtutils.py
    index 89175ca4ee60..5b86e441a1bc 100644
    --- i/qutebrowser/utils/qtutils.py
    +++ w/qutebrowser/utils/qtutils.py
    @@ -290,6 +290,20 @@ def savefile_open(
                 raise QtOSError(f, msg="Commit failed!")

    +def test_savefile_types() -> None:
    +    from typing_extensions import reveal_type
    +
    +    maybe_str_default = savefile_open("/tmp/string_file")
    +    # note: Revealed type is "contextlib._GeneratorContextManager[typing.IO[builtins.str]]"
    +    reveal_type(maybe_str_default)
    +    maybe_str_explicit = savefile_open("/tmp/string_file", binary=False)
    +    # note: Revealed type is "contextlib._GeneratorContextManager[typing.IO[builtins.str]]"
    +    reveal_type(maybe_str_explicit)
    +    maybe_bytes = savefile_open("/tmp/bytes_file", binary=True)
    +    # note: Revealed type is "contextlib._GeneratorContextManager[typing.IO[builtins.bytes]]"
    +    reveal_type(maybe_bytes)
    +
    +
     def qcolor_to_qsscolor(c: QColor) -> str:
         """Convert a QColor to a string that can be used in a QStyleSheet."""
         ensure_valid(c)

1 files changed, 2 insertions, 2 deletions

diff --git a/qutebrowser/utils/qtutils.py b/qutebrowser/utils/qtutils.py
index 363d5607a..89175ca4e 100644
--- a/qutebrowser/utils/qtutils.py
+++ b/qutebrowser/utils/qtutils.py
@@ -250,9 +250,9 @@ def savefile_open(
 @contextlib.contextmanager
 def savefile_open(
         filename: str,
-        binary: Literal[True] = ...,
+        binary: Literal[True],
         encoding: str = 'utf-8'
-) -> Iterator[IO[str]]:
+) -> Iterator[IO[bytes]]:
     ...