Fix/improve typing for qtutils.savefile_open

Contrary to what I thought at the time when initially writing this,
typing.AnyStr isn't just an alias for IO[str] | IO[bytes], but is actually a
TypeVar. As per the Python docs, it should be used when there are *multiple*
places where the types need to match:

    def concat(a: AnyStr, b: AnyStr) -> AnyStr:
        return a + b

What we do instead is somewhat akin to "def fun() -> T:", which mypy already
comments on:

    error: A function returning TypeVar should receive at least one argument
    containing the same TypeVar  [type-var]
        def t() -> T:

Not quite sure why it doesn't in this case, or why it now raises an additional
error (possibly the new inferrence code or something?). Either way, with this
commit the annotations are now more correctly using Union[IO[str], IO[bytes]],
including typing.Literal overloads so that mypy actually knows what specific
type will be returned by a call.

qutebrowser/utils/qtutils.py

@@ -18,8 +18,8 @@ import enum
 import pathlib
 import operator
 import contextlib
-from typing import (Any, AnyStr, TYPE_CHECKING, BinaryIO, IO, Iterator,
-                    Optional, Union, Tuple, Protocol, cast, TypeVar)
+from typing import (Any, TYPE_CHECKING, BinaryIO, IO, Iterator, Literal,
+                    Optional, Union, Tuple, Protocol, cast, overload, TypeVar)
 
 from qutebrowser.qt import machinery, sip
 from qutebrowser.qt.core import (qVersion, QEventLoop, QDataStream, QByteArray,
@@ -236,12 +236,32 @@ def deserialize_stream(stream: QDataStream, obj: _QtSerializableType) -> None:
     check_qdatastream(stream)
 
 
+@overload
+@contextlib.contextmanager
+def savefile_open(
+        filename: str,
+        binary: Literal[False] = ...,
+        encoding: str = 'utf-8'
+) -> Iterator[IO[str]]:
+    ...
+
+
+@overload
+@contextlib.contextmanager
+def savefile_open(
+        filename: str,
+        binary: Literal[True] = ...,
+        encoding: str = 'utf-8'
+) -> Iterator[IO[str]]:
+    ...
+
+
 @contextlib.contextmanager
 def savefile_open(
         filename: str,
         binary: bool = False,
         encoding: str = 'utf-8'
-) -> Iterator[IO[AnyStr]]:
+) -> Iterator[Union[IO[str], IO[bytes]]]:
     """Context manager to easily use a QSaveFile."""
     f = QSaveFile(filename)
     cancelled = False
@@ -253,7 +273,7 @@ def savefile_open(
         dev = cast(BinaryIO, PyQIODevice(f))
 
         if binary:
-            new_f: IO[Any] = dev  # FIXME:mypy Why doesn't AnyStr work?
+            new_f: Union[IO[str], IO[bytes]] = dev
         else:
             new_f = io.TextIOWrapper(dev, encoding=encoding)