capture: improve `SysCapture`/`FDCapture` typing

Instead of `SysCapture`/`FDCapture` inheriting from
`SysCaptureBinary`/`FDCaptureBinary`, have both inherit from a common
`SysCaptureBase`/`FDCaptureBase`. This fixes a Liskov substitution
violation.

src/_pytest/capture.py

@@ -278,13 +278,12 @@ class NoCapture:
     __init__ = start = done = suspend = resume = lambda *args: None
 
 
-class SysCaptureBinary:
-
-    EMPTY_BUFFER = b""
-
-    def __init__(self, fd: int, tmpfile=None, *, tee: bool = False) -> None:
+class SysCaptureBase:
+    def __init__(
+        self, fd: int, tmpfile: Optional[TextIO] = None, *, tee: bool = False
+    ) -> None:
         name = patchsysdict[fd]
-        self._old = getattr(sys, name)
+        self._old: TextIO = getattr(sys, name)
         self.name = name
         if tmpfile is None:
             if name == "stdin":
@@ -324,14 +323,6 @@ class SysCaptureBinary:
         setattr(sys, self.name, self.tmpfile)
         self._state = "started"
 
-    def snap(self):
-        self._assert_state("snap", ("started", "suspended"))
-        self.tmpfile.seek(0)
-        res = self.tmpfile.buffer.read()
-        self.tmpfile.seek(0)
-        self.tmpfile.truncate()
-        return res
-
     def done(self) -> None:
         self._assert_state("done", ("initialized", "started", "suspended", "done"))
         if self._state == "done":
@@ -353,36 +344,43 @@ class SysCaptureBinary:
         setattr(sys, self.name, self.tmpfile)
         self._state = "started"
 
-    def writeorg(self, data) -> None:
+
+class SysCaptureBinary(SysCaptureBase):
+    EMPTY_BUFFER = b""
+
+    def snap(self) -> bytes:
+        self._assert_state("snap", ("started", "suspended"))
+        self.tmpfile.seek(0)
+        res = self.tmpfile.buffer.read()
+        self.tmpfile.seek(0)
+        self.tmpfile.truncate()
+        return res
+
+    def writeorg(self, data: bytes) -> None:
         self._assert_state("writeorg", ("started", "suspended"))
         self._old.flush()
         self._old.buffer.write(data)
         self._old.buffer.flush()
 
 
-class SysCapture(SysCaptureBinary):
-    EMPTY_BUFFER = ""  # type: ignore[assignment]
+class SysCapture(SysCaptureBase):
+    EMPTY_BUFFER = ""
 
-    def snap(self):
+    def snap(self) -> str:
+        self._assert_state("snap", ("started", "suspended"))
+        assert isinstance(self.tmpfile, CaptureIO)
         res = self.tmpfile.getvalue()
         self.tmpfile.seek(0)
         self.tmpfile.truncate()
         return res
 
-    def writeorg(self, data):
+    def writeorg(self, data: str) -> None:
         self._assert_state("writeorg", ("started", "suspended"))
         self._old.write(data)
         self._old.flush()
 
 
-class FDCaptureBinary:
-    """Capture IO to/from a given OS-level file descriptor.
-
-    snap() produces `bytes`.
-    """
-
-    EMPTY_BUFFER = b""
-
+class FDCaptureBase:
     def __init__(self, targetfd: int) -> None:
         self.targetfd = targetfd
 
@@ -447,14 +445,6 @@ class FDCaptureBinary:
         self.syscapture.start()
         self._state = "started"
 
-    def snap(self):
-        self._assert_state("snap", ("started", "suspended"))
-        self.tmpfile.seek(0)
-        res = self.tmpfile.buffer.read()
-        self.tmpfile.seek(0)
-        self.tmpfile.truncate()
-        return res
-
     def done(self) -> None:
         """Stop capturing, restore streams, return original capture file,
         seeked to position zero."""
@@ -487,22 +477,38 @@ class FDCaptureBinary:
         os.dup2(self.tmpfile.fileno(), self.targetfd)
         self._state = "started"
 
-    def writeorg(self, data):
+
+class FDCaptureBinary(FDCaptureBase):
+    """Capture IO to/from a given OS-level file descriptor.
+
+    snap() produces `bytes`.
+    """
+
+    EMPTY_BUFFER = b""
+
+    def snap(self) -> bytes:
+        self._assert_state("snap", ("started", "suspended"))
+        self.tmpfile.seek(0)
+        res = self.tmpfile.buffer.read()
+        self.tmpfile.seek(0)
+        self.tmpfile.truncate()
+        return res
+
+    def writeorg(self, data: bytes) -> None:
         """Write to original file descriptor."""
         self._assert_state("writeorg", ("started", "suspended"))
         os.write(self.targetfd_save, data)
 
 
-class FDCapture(FDCaptureBinary):
+class FDCapture(FDCaptureBase):
     """Capture IO to/from a given OS-level file descriptor.
 
     snap() produces text.
     """
 
-    # Ignore type because it doesn't match the type in the superclass (bytes).
-    EMPTY_BUFFER = ""  # type: ignore
+    EMPTY_BUFFER = ""
 
-    def snap(self):
+    def snap(self) -> str:
         self._assert_state("snap", ("started", "suspended"))
         self.tmpfile.seek(0)
         res = self.tmpfile.read()
@@ -510,9 +516,11 @@ class FDCapture(FDCaptureBinary):
         self.tmpfile.truncate()
         return res
 
-    def writeorg(self, data):
+    def writeorg(self, data: str) -> None:
         """Write to original file descriptor."""
-        super().writeorg(data.encode("utf-8"))  # XXX use encoding of original stream
+        self._assert_state("writeorg", ("started", "suspended"))
+        # XXX use encoding of original stream
+        os.write(self.targetfd_save, data.encode("utf-8"))
 
 
 # MultiCapture