feat: make Python scripts compatible with both Python 2 and 3

Update print statements to function syntax, add from __future__ import print_function in test.py, change shebangs to /usr/bin/env python, replace str.translate with re.sub for anchor sanitization in generate_docs.py, and decode subprocess output to utf-8 in test.py for cross-version string handling.

script/generate_builtins.py

@@ -1,4 +1,5 @@
 #!/usr/bin/env python
+
 import glob
 import os.path
 import re
@@ -33,7 +34,7 @@ def copy_builtin(filename):
   with open("src/wren_" + name + ".c", "w") as f:
     f.write(c_source)
 
-  print name
+  print(name)
 
 
 for f in glob.iglob("builtin/*.wren"):