Fix compilation of python/python.c for Python 3.9

Fix compilation of python/python.c for Python 3.9

This commit fixes a compilation warning/error when building GDB
with Python 3.9:

g++ -x c++  -O2 -g -pipe -Wall -Werror=format-security -Wp,-D_FORTIFY_SOURCE=2 -Wp,-D_GLIBCXX_ASSERTIONS -fexceptions -fstack-protector-strong -grecord-gcc-switches -specs=/usr/lib/rpm/redhat/redhat-hardened-cc1 -specs=/usr/lib/rpm/redhat/redhat-annobin-cc1 -m64 -mtune=generic -fasynchronous-unwind-tables -fstack-clash-protection -fcf-protection  -DDNF_DEBUGINFO_INSTALL   -I. -I../../gdb -I../../gdb/config -DLOCALEDIR="\"/usr/share/locale\"" -DHAVE_CONFIG_H -I../../gdb/../include/opcode   -I../bfd -I../../gdb/../bfd -I../../gdb/../include -I../libdecnumber -I../../gdb/../libdecnumber  -I../../gdb/../gnulib/import -I../gnulib/import  -DTUI=1    -I/usr/include/guile/2.0 -pthread  -I/usr/include/python3.9 -I/usr/include/python3.9  -I../../gdb/.. -pthread -Wall -Wpointer-arith -Wno-unused -Wunused-value -Wunused-variable -Wunused-function -Wno-switch -Wno-char-subscripts -Wempty-body -Wunused-but-set-parameter -Wunused-but-set-variable -Wno-sign-compare -Wno-error=maybe-uninitialized -Wno-mismatched-tags -Wsuggest-override -Wimplicit-fallthrough=3 -Wduplicated-cond -Wshadow=local -Wdeprecated-copy -Wdeprecated-copy-dtor -Wredundant-move -Wformat -Wformat-nonliteral -Wno-unused -Werror -c -o ser-tcp.o -MT ser-tcp.o -MMD -MP -MF ./.deps/ser-tcp.Tpo ../../gdb/ser-tcp.c
../../gdb/python/python.c: In function 'bool do_start_initialization()':
../../gdb/python/python.c:1621:23: error: 'void PyEval_InitThreads()' is deprecated [-Werror=deprecated-declarations]
 1621 |   PyEval_InitThreads ();
      |                       ^
In file included from /usr/include/python3.9/Python.h:141,
                 from ../../gdb/python/python-internal.h:86,
                 from ../../gdb/python/python.c:92:
/usr/include/python3.9/ceval.h:132:37: note: declared here
  132 | Py_DEPRECATED(3.9) PyAPI_FUNC(void) PyEval_InitThreads(void);
      |                                     ^~~~~~~~~~~~~~~~~~

Information about the deprecated function can be found here:

https://docs.python.org/3.9/whatsnew/3.9.html#deprecated

Specifically, with regard to PyEval_InitThreads(), it says:

    The PyEval_InitThreads() and PyEval_ThreadsInitialized() functions
    are now deprecated and will be removed in Python 3.11.  Calling
    PyEval_InitThreads() now does nothing.  The GIL is initialized by
    Py_Initialize() since Python 3.7.  (Contributed by Victor Stinner
    in bpo-39877.)

I chose to disable the call with a #if test using PY_VERSION_HEX.
There is precedent for use of PY_VERSION_HEX; it's used in two places
in python-internal.h.  I noticed that under certain circumstances
python-internal.h defines PyEval_InitThreads to be nothing, which
accomplishes the same thing.  I considered doing something similar for
this case, but decided against it because, at some point in the future,
the presence of PyEval_InitThreads() without some explanation will be
confusing to a reader who won't be able to find PyEval_InitThreads in
the current (future for us) Python API.  IMO, use of the #if along
with an accompanying comment seemed more straightforward.

gdb/ChangeLog:

	* python/python.c (do_start_initialization): Don't call
	PyEval_InitThreads for Python 3.9 and beyond.

Change-Id: I0679fc10b6b76761a99538568f13188c6d8014e0