docs/autogen_shortcuts.py: support Python 3.8

In 64e72a955 (Restore shortcuts in documentation, define identifiers,
2023-01-08), some typing annotations were added to
docs/autogen_shortcuts.py using the builtin container type 'list'.  This
feature is only available starting in Python 3.9 [1], but setup.cfg
lists Python 3.8 as the earliest supported Python version. This leads to
a failing documentation build in a Python 3.8 virtual environment.

Fix this by using the capitalized name 'List' from the 'typing' module
to keep Python 3.8 compatibility.

[1] https://docs.python.org/3/whatsnew/3.9.html#type-hinting-generics-in-standard-collections

docs/autogen_shortcuts.py

@@ -17,7 +17,7 b' from IPython.terminal.shortcuts.filters import KEYBINDING_FILTERS'
 @dataclass
 class Shortcut:
     #: a sequence of keys (each element on the list corresponds to pressing one or more keys)
-    keys_sequence: list[str]
+    keys_sequence: List[str]
     filter: str
 
 
@@ -146,7 +146,7 b' def format_prompt_keys(keys: str, add_alternatives=True) -> str:'
         escaped = key.replace("\\", "\\\\")
         return f":kbd:`{escaped}`"
 
-    keys_to_press: list[str]
+    keys_to_press: List[str]
 
     prefixes = {
         "c-s-": [to_rst("ctrl"), to_rst("shift")],