output/vt100: Cope with stdout not having a working fileno()

When the application is using an io.StringIO(), its isatty() properly
returns False but its fileno() raises io.UnsupportedOperation, so we
have to cope with it.

Fixes #1136

Author: sthibaul

prompt_toolkit/output/vt100.py

@@ -8,6 +8,7 @@
 """
 import array
 import errno
+import io
 import os
 import sys
 from typing import (
@@ -452,16 +453,21 @@ def from_pty(
         (This will take the dimensions by reading the pseudo
         terminal attributes.)
         """
+        fd: Optional[int]
         # Normally, this requires a real TTY device, but people instantiate
         # this class often during unit tests as well. For convenience, we print
         # an error message, use standard dimensions, and go on.
-        fd = stdout.fileno()
+        try:
+            fd = stdout.fileno()
+        except io.UnsupportedOperation:
+            fd = None
 
-        if not stdout.isatty() and fd not in cls._fds_not_a_terminal:
+        if not stdout.isatty() and (fd is None or fd not in cls._fds_not_a_terminal):
             msg = "Warning: Output is not a terminal (fd=%r).\n"
             sys.stderr.write(msg % fd)
             sys.stderr.flush()
-            cls._fds_not_a_terminal.add(fd)
+            if fd is not None:
+                cls._fds_not_a_terminal.add(fd)
 
         def get_size() -> Size:
             # If terminal (incorrectly) reports its size as 0, pick a