[gdb/testsuite] Require python in gdb.server/server-kill-python.exp

I came across this when running test-case gdb.server/server-kill-python.exp
with a gdb configured without python:
...
builtin_spawn gdb -nw -nx -data-directory data-directory -iex set height 0 \
  -iex set width 0 -quiet -iex set height 0 -iex set width 0 \
  -ex source outputs/gdb.server/server-kill-python/file1.py^M
FAIL: gdb.server/server-kill-python.exp: ensure inferior is running
Executing on target: kill -9 28535    (timeout = 300)
builtin_spawn -ignore SIGHUP kill -9 28535^M
file1.py:1: Error in sourced command file:^M
Undefined command: "import".  Try "help".^M
...

Fix this by testing for python support in the test-case.

Tested on aarch64-linux (with python support disabled) and x86_64-linux (with
python support enabled).

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