[gdb/python] Don't import curses.ascii module unless necessary

I ran into a failure in test-case gdb.python/py-missing-debug.exp with python
3.6, which was fixed by commit 7db795bc ("gdb/python: remove use of
str.isascii()").

However, I subsequently ran into a failure with python 3.11:
...
(gdb) PASS: $exp: initial checks: debug info no longer found
source py-missing-debug.py^M
Traceback (most recent call last):^M
  File "py-missing-debug.py", line 17, in <module>^M
    from gdb.missing_debug import MissingDebugHandler^M
  File "missing_debug.py", line 21, in <module>^M
    from curses.ascii import isascii, isalnum^M
  File "/usr/lib64/python3.11/_import_failed/curses.py", line 16, in <module>^M
    raise ImportError(f"""Module '{failed_name}' is not installed.^M
ImportError: Module 'curses' is not installed.^M
Use:^M
  sudo zypper install python311-curses^M
to install it.^M
(gdb) FAIL: $exp: source python script
...

Apparently I have the curses module installed for 3.6, but not 3.11.

I could just install it, but the test-case worked fine with 3.11 before commit
7db795bc.

Fix this by only using the curses module when necessary, for python <= 3.7.

Tested on x86_64-linux, with both python 3.6 and 3.11.