Fixed #29774 -- Fixed django-admin shell hang on startup.

sys.stdin.read() blocks waiting for EOF in shell.py which will
likely never come if the user provides input on stdin via the
keyboard before the shell starts. Added check for a tty to
skip reading stdin if it's not present.

This still allows piping of code into the shell (which should
have no TTY and should have an EOF) but also doesn't cause it
to hang if multi-line input is provided.

django/core/management/commands/shell.py

@@ -88,7 +88,7 @@ class Command(BaseCommand):
 
         # Execute stdin if it has anything to read and exit.
         # Not supported on Windows due to select.select() limitations.
-        if sys.platform != 'win32' and select.select([sys.stdin], [], [], 0)[0]:
+        if sys.platform != 'win32' and not sys.stdin.isatty() and select.select([sys.stdin], [], [], 0)[0]:
             exec(sys.stdin.read())
             return