[gdb/testsuite] Fix gdb.multi/pending-bp.exp without python support

[gdb/testsuite] Fix gdb.multi/pending-bp.exp without python support

With a gdb build without python support and test-case
gdb.multi/pending-bp.exp, I run into:
...
(gdb) python bp=[b for b in gdb.breakpoints() if b.number == 5][0]^M
Python scripting is not supported in this copy of GDB.^M
(gdb) FAIL: $exp: py_test_toggle_thread: find Python gdb.Breakpoint object
...

Fix this by requiring python support for part of the test-case.

Tested on aarch64-linux.

Reviewed-By: Keith Seitz <keiths@redhat.com>