Use os.devnull + do not check if system exists for os module

dvc/utils/__init__.py

@@ -423,5 +423,5 @@ def resolve_output(inp, out):
 
 
 def is_exec_found(exec_name):
-    cmd = "({}) 2>/dev/null".format(exec_name)
-    return hasattr(os, "system") and os.system(cmd) == 0
+    cmd = "({}) 2>{}".format(exec_name, os.devnull)
+    return os.system(cmd) == 0

tests/func/test_utils.py

@@ -91,7 +91,7 @@ def test_makedirs_permissions(tmpdir):
 
 
 def test_is_exec_found_returns_true_when_program_exists():
-    result = utils.is_exec_found("python")
+    result = utils.is_exec_found("echo")
     assert result is True