gdb/python: avoid depending on the curses library

The commit:

  commit 29c70787
  Date:   Sun Sep 8 07:46:09 2024 +0200

      [gdb/testsuite] Handle missing curses in gdb.python/py-missing-debug.exp

Highlighted that in some cases we might be running on a system with an
older version of Python (earlier than 3.7), and on a system for which
the curses library has not been installed.

In these circumstances the gdb.missing_debug module will not load as
it uses curses to provide isalnum() and isascii() functions.

To avoid this problem I propose that we copy the isalnum() and
isascii() from the Python curses library.  These functions are
basically trivial and removing the curses dependency means GDB will
work in more cases without increasing its dependencies.

I did consider keeping the uses of curses and only having the function
definitions be a fallback for when the curses library failed to load,
but this felt like overkill.  The function definitions are both tiny
and I think "obvious" given their specifications, so I figure we might
as well just use our own definitions if they are not available as
builtin methods on the str class.

For testing I changed this line:

  if sys.version_info >= (3, 7):

to

  if sys.version_info >= (3, 7) and False:

then reran gdb.python/py-missing-debug.exp, there were no failures.

Approved-By: Tom de Vries <tdevries@suse.de>