gdb/python: remove use of str.isascii()

This commit:

  commit 8f6c452b
  Date:   Sun Oct 15 22:48:42 2023 +0100

      gdb: implement missing debug handler hook for Python

introduced a use of str.isascii(), which was only added in Python 3.7.

This commit switches to use curses.ascii.isascii(), as this was
available in 3.6.

The same is true for str.isalnum(), which is replaced with
curses.ascii.isalnum().

There should be no user visible changes after this commit.