Fix leak detected in python.c initialization code.

Fix leak detected in python.c initialization code.

Valgrind reports the below leak.
Make the variable progname_copy static, so that Valgrind continues
to find a pointer to the memory given to Python.
Note that the comment in do_start_initialization and the Python documentation
indicates that the progname given to Py_SetProgramName cannot be freed.
However, in Python 3.7.4, Py_SetProgramName does:
void
Py_SetProgramName(const wchar_t *program_name)
{
    ...
    PyMem_RawFree(_Py_path_config.program_name);
    _Py_path_config.program_name = _PyMem_RawWcsdup(program_name);

So, it looks like 3.7.4 Python duplicates its argument, which explains
the leak found by Valgrind.
It looks better to respect the doc and not have GDB freeing the string
given to Py_SetProgramName, and avoid the leak error by declaring
the progname_copy static.
This will work with Python versions that really use this string without
duplicating it, and avoids a leak report for Python version that duplicates
it.

==4023== 200 bytes in 1 blocks are definitely lost in loss record 4,545 of 7,116^M
==4023==    at 0x4C29F33: malloc (vg_replace_malloc.c:307)^M
==4023==    by 0x446D27: xmalloc (alloc.c:60)^M
==4023==    by 0x657C77: do_start_initialization (python.c:1610)^M
==4023==    by 0x657C77: _initialize_python() (python.c:1823)^M
==4023==    by 0x75FE24: initialize_all_files() (init.c:231)^M
==4023==    by 0x708A94: gdb_init(char*) (top.c:2242)^M
==4023==    by 0x5E7460: captured_main_1 (main.c:857)^M
==4023==    by 0x5E7460: captured_main (main.c:1161)^M
==4023==    by 0x5E7460: gdb_main(captured_main_args*) (main.c:1186)^M
==4023==    by 0x4122D4: main (gdb.c:32)^M

gdb/ChangeLog
2019-09-09  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* python/python.c (do_start_initialization): Make progname_copy static,
	to avoid a leak report.