dependencies: vendor "pyperclip" pkg, used by text gui

commit 43ab4a779a introduced "pyperclip" as a dependency for the prototype "text" gui.

pyperclip is small enough for us to vendor it, and looks mature/stable enough so that it would ~never need updating.
This makes it easier to try out the text gui when running from source, or using the AppImage.

electrum/gui/text.py

@@ -8,7 +8,10 @@ import getpass
 from typing import TYPE_CHECKING, Optional
 
 # 3rd-party dependency:
-import pyperclip
+try:
+    import pyperclip
+except ImportError:  # only use vendored lib as fallback, to allow Linux distros to bring their own
+    from electrum._vendor import pyperclip
 
 import electrum
 from electrum.gui import BaseElectrumGui