bpo-31955: Fix distutils CCompiler.set_executable() for Unicode (GH-4316)

Fix CCompiler.set_executable() of distutils to handle properly Unicode strings.
diff --git a/Lib/distutils/ccompiler.py b/Lib/distutils/ccompiler.py
index 62506a6..88a910c 100644
--- a/Lib/distutils/ccompiler.py
+++ b/Lib/distutils/ccompiler.py
@@ -160,7 +160,7 @@
             self.set_executable(key, args[key])
 
     def set_executable(self, key, value):
-        if isinstance(value, str):
+        if isinstance(value, basestring):
             setattr(self, key, split_quoted(value))
         else:
             setattr(self, key, value)