python: Conditionally provide py3 argument to SWIG

* lang/python/setup.py.in: Only call with -py3 when we run under python3
or higher.
--

If we ever remove the -builtin flag and leave the the -py3 flag, SWIG
will generate Python code which will be incompatible with Python 2,
because the py3 flag generates python3 code which is incompatible with
python2.

So we conditionally generate SWIG bindings with -py3.

Signed-off-by: Tobias Mueller <muelli@cryptobitch.de>

lang/python/setup.py.in

@@ -152,9 +152,10 @@ class BuildExtFirstHack(build):
         self.run_command('build_ext')
         build.run(self)
 
+py3 = [] if sys.version_info.major < 3 else ['-py3']
 swige = Extension("gpg._gpgme", ["gpgme.i", "helpers.c"],
-                  swig_opts = ['-py3', '-builtin', '-threads',
-                               '-outdir', 'gpg'] + extra_swig_opts,
+                  swig_opts = ['-threads', '-builtin',
+                               '-outdir', 'gpg'] + py3 + extra_swig_opts,
                   include_dirs = include_dirs,
                   define_macros = define_macros,
                   library_dirs = library_dirs,