fix: guard modal cursor import for prompt toolkit (#4758)

We shouldn't fail to start if the available PTK version doesn't support
modal cursor stuff.
(We should probably also make modal cursor stuff optional, but that can
happen later)

news/guard_cursor_shape.rst

@@ -0,0 +1,24 @@
+**Added:**
+
+* <news item>
+
+**Changed:**
+
+* <news item>
+
+**Deprecated:**
+
+* <news item>
+
+**Removed:**
+
+* <news item>
+
+**Fixed:**
+
+* Allow xonsh to start gracefully even if modal cursors aren't in the available
+  prompt_toolkit version
+
+**Security:**
+
+* <news item>

xonsh/ptk_shell/shell.py

@@ -7,7 +7,6 @@ from types import MethodType
 
 from prompt_toolkit import ANSI
 from prompt_toolkit.auto_suggest import AutoSuggestFromHistory
-from prompt_toolkit.cursor_shapes import ModalCursorShapeConfig
 from prompt_toolkit.enums import EditingMode
 from prompt_toolkit.formatted_text import PygmentsTokens, to_formatted_text
 from prompt_toolkit.history import ThreadedHistory
@@ -44,6 +43,13 @@ try:
 except ImportError:
     HAVE_SYS_CLIPBOARD = False
 
+try:
+    from prompt_toolkit.cursor_shapes import ModalCursorShapeConfig
+
+    HAVE_CURSOR_SHAPE = True
+except ImportError:
+    HAVE_CURSOR_SHAPE = False
+
 CAPITAL_PATTERN = re.compile(r"([a-z])([A-Z])")
 Token = _TokenType()
 
@@ -356,7 +362,7 @@ class PromptToolkitShell(BaseShell):
             for attr, val in self.get_lazy_ptk_kwargs():
                 prompt_args[attr] = val
 
-        if editing_mode == EditingMode.VI:
+        if editing_mode == EditingMode.VI and HAVE_CURSOR_SHAPE:
             prompt_args["cursor"] = ModalCursorShapeConfig()
         events.on_pre_prompt.fire()
         line = self.prompter.prompt(**prompt_args)