Replace b('str') with b'str' in Python 3

This avoids an encode() call for each of these constants.

Use a custom 2to3 fixer in setup.py to perform the conversion.

scripts/fix_b.py

@@ -0,0 +1,20 @@
+"""Fixer to change b('string') into b'string'."""
+# Author: Daniele Varrazzo
+
+import token
+from lib2to3 import fixer_base
+from lib2to3.pytree import Leaf
+
+class FixB(fixer_base.BaseFix):
+
+    PATTERN = """
+              power< wrapper='b' trailer< '(' arg=[any] ')' > rest=any* >
+              """
+
+    def transform(self, node, results):
+        arg = results['arg']
+        wrapper = results["wrapper"]
+        if len(arg) == 1 and arg[0].type == token.STRING:
+            b = Leaf(token.STRING, 'b' + arg[0].value, prefix=wrapper.prefix)
+            node.children = [ b ] + results['rest']
+

setup.py

@@ -58,6 +58,12 @@ try:
     from distutils.command.build_py import build_py_2to3 as build_py
 except ImportError:
     from distutils.command.build_py import build_py
+else:
+    # Configure distutils to run our custom 2to3 fixers as well
+    from lib2to3.refactor import get_fixers_from_package
+    build_py.fixer_names = get_fixers_from_package('lib2to3.fixes')
+    build_py.fixer_names.append('fix_b')
+    sys.path.insert(0, 'scripts')
 
 try:
     import configparser