Fix running command nodes on python 3

cibuildwheel/bashlex_eval.py

@@ -1,4 +1,4 @@
-import subprocess, shlex
+import subprocess, shlex, sys
 from collections import namedtuple
 import bashlex
 
@@ -60,8 +60,12 @@ def evaluate_word_node(node, context):
 def evaluate_command_node(node, context):
     words = [evaluate_node(part, context=context) for part in node.parts]
     command = ' '.join(words)
-    return subprocess.check_output(shlex.split(command), env=context.environment)
+    output = subprocess.check_output(shlex.split(command), env=context.environment)
 
+    if sys.version_info[0] >= 3:
+        return output.decode('utf8', 'replace')
+    else:
+        return output
 
 def evaluate_parameter_node(node, context):
     return context.environment.get(node.value, '')