[libc++] Fix GDB pretty printers when GDB uses Python 2.7

The gdb_pretty_printer_test.sh fails if GDB was built against Python 2.7
since Python 2 expects iterators to have a next() method rather than
using __next__. To make the pretty printers work with both Python 2 and 3
we can simply set next to __next__ in the iterator classes.

Python 2.7 support was removed in f46f93b4,
so this partially reverts that commit. While Python 2.7 is EOL, it
appears there are still many GDB installations that are linked against
Python 2.7, so we may want to keep this tiny amount of compat code
around for a while longer.

Without this commit the tests fails with errors such as:
```
GDB printed:
   u"std::tuple containingTypeError: iter() returned non-iterator of type '_Children'\n"
Value should match:
   u'std::tuple containing = {[1] = 2, [2] = 3, [3] = 4}'
```

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D117470