Python 2.7 compatibility fix: subprocess.DEVNULL

As reported in #14 (comment),
Python 2.7 does not have subprocess.DEVNULL. Use os.devnull instead.

python/treelite/common/compat.py

@@ -43,4 +43,11 @@ def _str_decode(string):
   def _str_encode(string):
     return string
 
+# define DEVNULL
+if PY3:
+  from subprocess import DEVNULL
+else:
+  import os
+  DEVNULL = open(os.devnull, 'r+b')
+
 __all__ = []

python/treelite/contrib/gcc.py

@@ -7,6 +7,7 @@
 import os
 import subprocess
 
+from ..common.compat import DEVNULL
 from ..common.util import TemporaryDirectory
 from .util import _create_shared_base, _libext, _shell
 
@@ -22,9 +23,7 @@ def _openmp_supported(toolchain):
     retcode = subprocess.call('{} -o {} {} -fopenmp'\
                               .format(toolchain, output, sfile),
                               shell=True,
-                              stdin=subprocess.DEVNULL,
-                              stdout=subprocess.DEVNULL,
-                              stderr=subprocess.DEVNULL)
+                              stdin=DEVNULL, stdout=DEVNULL, stderr=DEVNULL)
   return retcode == 0
 
 def _obj_ext():

python/treelite/contrib/util.py

@@ -5,7 +5,7 @@
 import subprocess
 from sys import platform as _platform
 from multiprocessing import cpu_count
-from ..common.compat import _str_decode, _str_encode
+from ..common.compat import _str_decode, _str_encode, DEVNULL
 from ..common.util import TreeliteError, lineno, log_info
 
 def _is_windows():
@@ -15,9 +15,7 @@ def _toolchain_exist_check(toolchain):
   if toolchain != 'msvc':
     retcode = subprocess.call('{} --version'.format(toolchain),
                               shell=True,
-                              stdin=subprocess.DEVNULL,
-                              stdout=subprocess.DEVNULL,
-                              stderr=subprocess.DEVNULL)
+                              stdin=DEVNULL, stdout=DEVNULL, stderr=DEVNULL)
     if retcode != 0:
       raise ValueError('Toolchain {} not found. '.format(toolchain) +
                       'Ensure that it is installed and that it is a variant ' +