Downgrade mypy for now

I believe we are being afflicted by this issue: python/mypy#16451
Although I'm not 100% sure because there is a lot going on in this
function and I haven't managed to grok it.

The mypy 1.7 release [notes][1.7] say you can disable the new type
inference by running `tox -e mypy-pyqt6 -- --old-type-inference` and
indeed mypy passes with that. So either our type hints are incorrect or
we are hitting a bug. Considering the inferred type hint has a `Never`
in it I'm leading toward it being a bug. So I'll bump the mypy version
down and hopefully next week the issue will be resolved.

The mypy output before this commit was:

    mypy-pyqt6: commands[0]> .tox/mypy-pyqt6/bin/python -m mypy --always-true=USE_PYQT6 --always-false=USE_PYQT5 --always-false=USE_PYSIDE6 --always-false=IS_QT5 --always-true=IS_QT6 --always-true=IS_PYQT --always-false=IS_PYSIDE qutebrowser
    qutebrowser/utils/qtutils.py:239: error: Argument 1 to "contextmanager" has incompatible type
    "Callable[[str, bool, str], Iterator[IO[AnyStr]]]"; expected "Callable[[str, bool, str], Iterator[IO[Never]]]"  [arg-type]
        @contextlib.contextmanager
         ^
    qutebrowser/misc/lineparser.py: note: In member "save" of class "LineParser":
    qutebrowser/misc/lineparser.py:168: error: Need type annotation for "f"  [var-annotated]
                    with qtutils.savefile_open(self._configfile, self._binary) as f:
                         ^
    qutebrowser/misc/lineparser.py: note: In member "save" of class "LimitLineParser":
    qutebrowser/misc/lineparser.py:226: error: Need type annotation for "f"  [var-annotated]
                with qtutils.savefile_open(self._configfile, self._binary) as f:
                     ^
    qutebrowser/config/configfiles.py: note: In member "_save" of class "YamlConfig":
    qutebrowser/config/configfiles.py:292: error: Need type annotation for "f"  [var-annotated]
                with qtutils.savefile_open(self._filename) as f:
                     ^
    qutebrowser/misc/sessions.py: note: In member "save" of class "SessionManager":
    qutebrowser/misc/sessions.py:343: error: Need type annotation for "f"  [var-annotated]
                    with qtutils.savefile_open(path) as f:

[1.7]: https://mypy-lang.blogspot.com/2023/11/mypy-17-released.html

misc/requirements/requirements-mypy.txt

@@ -6,7 +6,7 @@ importlib-resources==6.1.1
 Jinja2==3.1.2
 lxml==4.9.3
 MarkupSafe==2.1.3
-mypy==1.7.0
+mypy==1.6.1
 mypy-extensions==1.0.0
 pluggy==1.3.0
 Pygments==2.16.1