feat: Add OSC 52 fallback backend for headless environments

[iloveitaly]

This PR introduces a fallback clipboard backend using the OSC 52 terminal escape sequence. I'm using it in https://github.com/iloveitaly/ipython-copy

Currently, running pyperclip on a headless Linux server, over SSH, or inside a Docker container fails if GUI clipboard utilities (like xclip or wl-clipboard) aren't installed. This PR allows pyperclip.copy() to work automatically in these environments by communicating directly with the user's terminal emulator.

Details:

• Automatic Fallback: If no GUI clipboard utilities are found and a TTY is detected, the library now automatically defaults to OSC 52.
• Clean Output: Writes the escape sequence directly to /dev/tty (falling back to stderr) to prevent the payload from polluting standard output during script piping or redirection.
• Multiplexer Support: Formats the sequence correctly when running inside tmux or screen.
• Safety Constraints:
  • pyperclip.paste() is explicitly disabled to prevent indefinite hangs, as most terminal emulators block OSC 52 read requests by default due to security risks.
  • A hard payload limit of ~75 KB is enforced to prevent overwhelming or crashing the host terminal emulator.

Fixes #288
Fixes #161
Fixes #140

---

here's a script that helped me test this OSC stuff in various terminal environments

import os
import sys
import base64

def osc52_copy(text: str):
    """
    Attempts to copy text to the local system clipboard using OSC 52.
    Handles wrapping for terminal multiplexers like tmux and screen.
    """
    # 1. Base64 encode the payload (required by the OSC 52 protocol)
    b64_payload = base64.b64encode(text.encode('utf-8')).decode('ascii')
    
    # 2. Standard OSC 52 sequence: ESC ] 52 ; c ; <payload> BEL
    # \x1b is ESC, \x07 is BEL (the standard terminator)
    osc_seq = f"\x1b]52;c;{b64_payload}\x07"
    
    # 3. Handle Multiplexers (tmux/screen)
    # Multiplexers often 'swallow' escape codes unless they are wrapped 
    # in a special pass-through sequence.
    term = os.environ.get("TERM", "").lower()
    
    if "TMUX" in os.environ:
        # tmux wrap: ESC P tmux ; ESC <sequence> ESC \
        final_seq = f"\x1bPtmux;\x1b{osc_seq}\x1b\\"
        print("Detected: tmux (using pass-through wrapping)")
    elif term.startswith("screen"):
        # GNU Screen wrap: ESC P <sequence> ESC \
        final_seq = f"\x1bP{osc_seq}\x1b\\"
        print("Detected: GNU Screen (using pass-through wrapping)")
    else:
        # Standard terminal
        final_seq = osc_seq
        print(f"Detected: Standard Terminal (TERM={term})")

    # 4. Write directly to stdout and flush immediately
    sys.stdout.write(final_seq)
    sys.stdout.flush()

if __name__ == "__main__":
    test_string = "Hello from OSC 52! (Copied at " + os.popen('date').read().strip() + ")"
    
    print("-" * 50)
    print("OSC 52 Clipboard Test Script")
    print("-" * 50)
    print(f"Attempting to copy: '{test_string}'")
    
    osc52_copy(test_string)
    
    print("\n" + "=" * 50)
    print("FINISHED: The escape sequence has been sent to your terminal.")
    print("=" * 50)
    print("HOW TO VERIFY:")
    print("1. Switch to a DIFFERENT application (e.g., your browser, Notes, or a text editor).")
    print("2. Use your standard 'Paste' command (Cmd+V or Ctrl+V).")
    print("-" * 50)
    print("NOTE: If it doesn't work, your terminal emulator (e.g., iTerm2, Kitty, Windows Terminal)")
    print("might have 'Allow OSC 52' disabled in its settings.")
    print("-" * 50)