Fix "return"->"enter" mapping in key names.

All backends previously normalized both the return key and the (numpad)
enter to "enter" (see e.g. the presence of two "enter" entries in the
old FigureCanvas{GTK3,Tk}.keyvald . When revamping the gtk and tk key
names lookup I accidentally normalized both to "return" instead, so fix
that.

Test with
```
gcf().canvas.mpl_connect("key_press_event", lambda e: print(e.key))
```

Author: anntzer

lib/matplotlib/cbook/__init__.py

@@ -2308,7 +2308,7 @@ def _unikey_or_keysym_to_mplkey(unikey, keysym):
     if key.endswith(("_l", "_r")):  # alt_l, ctrl_l, shift_l.
         key = key[:-2]
     key = {
-        "enter": "return",
+        "return": "enter",
         "prior": "pageup",  # Used by tk.
         "next": "pagedown",  # Used by tk.
     }.get(key, key)