Improve open overloads when mode is a literal union

As pointed out by @gvanrossum in python/typing#1096

Improves type inference in cases when we know that mode is
OpenBinaryMode, but don't know anything more specific:
```
def my_open(name: str, write: bool):
    mode: Literal['rb', 'wb'] = 'wb' if write else 'rb'
    with open(name, mode) as f:
        reveal_type(f)  # previously typing.IO[Any], now typing.BinaryIO
```

You may be tempted into thinking this is some limitation of type
checkers. mypy does in fact have logic for detecting if we match
multiple overloads and union-ing up the return types of matched
overloads. The problem is the last overload interferes with this logic.
That is, if you remove the fallback overload (prior to this PR), you'd get
"Union[io.BufferedReader, io.BufferedWriter]" in the above example.

stdlib/builtins.pyi

@@ -1308,7 +1308,7 @@ def open(
 def open(
     file: _OpenFile,
     mode: OpenBinaryMode,
-    buffering: int,
+    buffering: int = ...,
     encoding: None = ...,
     errors: None = ...,
     newline: None = ...,

stdlib/importlib/abc.pyi

@@ -158,7 +158,7 @@ if sys.version_info >= (3, 9):
         @overload
         @abstractmethod
         def open(
-            self, mode: OpenBinaryMode, buffering: int, encoding: None = ..., errors: None = ..., newline: None = ...
+            self, mode: OpenBinaryMode, buffering: int = ..., encoding: None = ..., errors: None = ..., newline: None = ...
         ) -> BinaryIO: ...
         # Fallback if mode is not specified
         @overload

stdlib/os/__init__.pyi

@@ -568,7 +568,7 @@ def fdopen(
 def fdopen(
     fd: int,
     mode: OpenBinaryMode,
-    buffering: int,
+    buffering: int = ...,
     encoding: None = ...,
     errors: None = ...,
     newline: None = ...,

stdlib/pathlib.pyi

@@ -147,7 +147,7 @@ class Path(PurePath):
     # Buffering cannot be determined: fall back to BinaryIO
     @overload
     def open(
-        self, mode: OpenBinaryMode, buffering: int, encoding: None = ..., errors: None = ..., newline: None = ...
+        self, mode: OpenBinaryMode, buffering: int = ..., encoding: None = ..., errors: None = ..., newline: None = ...
     ) -> BinaryIO: ...
     # Fallback if mode is not specified
     @overload

stubs/aiofiles/aiofiles/threadpool/__init__.pyi

@@ -86,7 +86,7 @@ def open(
 def open(
     file: _OpenFile,
     mode: OpenBinaryMode,
-    buffering: int,
+    buffering: int = ...,
     encoding: None = ...,
     errors: None = ...,
     newline: None = ...,