Add rudimentary support for _tkinter in GraalPy

This commit adds the _tkinter module to the list of built-in modules to allow use of tkinter.
Before use, the bindings need to be built using tklib_build.py.
The code was modified from existing code in PyPy's main branch.

Right now, opening a window under Linux works, but still crashes on interacting with UI elements such as buttons.
It seems like there is a bug in the `cffi` module causing a SegFault.
macOS complains about other threads accessing the main window and the build script has some trouble finding the correct library for Tcl and Tk, as macOS comes with an older version preinstalled.
Windows was not tested.

Co-authored-by: Margarete Dippel <margarete.dippel@student.hpi.de>
Co-authored-by: Tim Felgentreff <tim.felgentreff@oracle.com>

graalpython/lib-python/3/_tkinter/__init__.py

@@ -0,0 +1,55 @@
+# _tkinter package -- low-level interface to libtk and libtcl.
+#
+# This is an internal module, applications should "import tkinter" instead.
+#
+# This version is based PyPy which itself is based on cffi, and is a translation of _tkinter.c
+# from CPython, version 2.7.4.
+
+import sys
+
+class TclError(Exception):
+    pass
+
+from .tklib_cffi import ffi as tkffi, lib as tklib
+
+from .app import TkApp
+from .tclobj import TclObject as Tcl_Obj
+from .app import FromTclString, ToTCLString
+
+TK_VERSION = FromTclString(tkffi.string(tklib.get_tk_version()))
+TCL_VERSION = FromTclString(tkffi.string(tklib.get_tcl_version()))
+
+READABLE = tklib.TCL_READABLE
+WRITABLE = tklib.TCL_WRITABLE
+EXCEPTION = tklib.TCL_EXCEPTION
+DONT_WAIT = tklib.TCL_DONT_WAIT
+
+def create(screenName=None, baseName=None, className=None,
+           interactive=False, wantobjects=False, wantTk=True,
+           sync=False, use=None):
+    return TkApp(screenName, baseName, className,
+                 interactive, wantobjects, wantTk, sync, use)
+
+def dooneevent(flags=0):
+    return tklib.Tcl_DoOneEvent(flags)
+
+
+def _flatten(item):
+    def _flatten1(output, item, depth):
+        if depth > 1000:
+            raise ValueError("nesting too deep in _flatten")
+        if not isinstance(item, (list, tuple)):
+            raise TypeError("argument must be sequence")
+        # copy items to output tuple
+        for o in item:
+            if isinstance(o, (list, tuple)):
+                _flatten1(output, o, depth + 1)
+            elif o is not None:
+                output.append(o)
+
+    result = []
+    _flatten1(result, item, 0)
+    return tuple(result)
+
+# Encoding is not specified explicitly, but "must be passed argv[0]" sounds like a simple conversion to raw bytes.
+tklib.Tcl_FindExecutable(ToTCLString(sys.executable))