tools/scons: switch to Python 3

tools/scons: switch to Python 3

Build tested on ath79 with following packages from packages feed which
build depends on scons. None of them build anymore as it seems, that the
SConscripts are written for Python2.

 * packages/net/iotivity (KO, doesn't build even with latest 1.3.1 release)

   SyntaxError: invalid syntax
    File "/openwrt/build_dir/target-mips_24kc_musl/iotivity-1.2.1/build_common/SConscript", line 40
      print "\nError: Current system (%s) isn't supported\n" % host

   LookupError: unknown encoding: string_escape:
    File "/home/petr/testing/openwrt/build_dir/target-mips_24kc_musl/iotivity-1.3.1/SConstruct", line 28:
      SConscript('build_common/SConscript')

 * packages/net/smartsnmpd (KO, seems dead, no commit since 2015):

   SyntaxError: Missing parentheses in call to 'print'
    File "/openwrt/build_dir/target-mips_24kc_musl/smartsnmpd-2014-08-13/SConstruct", line 156
      print "Can't find liblua or liblua5.1!"

 * packages/utils/gpsd (KO, doesn't build even with latest 3.18.1 release):

   AttributeError: 'list' object has no attribute 'keys':
     File "/openwrt/build_dir/target-mips_24kc_musl/gpsd-3.18.1/SConstruct", line 1758:
       all_manpages = list(base_manpages.keys()) + list(python_manpages.keys())

Signed-off-by: Petr Štetiar <ynezz@true.cz>