Introduce importlib.abc. The module contains various ABCs related to imports

(mostly stuff specified by PEP 302). There are two ABCs, PyLoader and
PyPycLoader, which help with implementing source and source/bytecode loaders by
implementing load_module in terms of other methods. This removes a lot of
gritty details loaders typically have to worry about.

Lib/importlib/test/source/util.py

@@ -1,5 +1,6 @@
 from .. import util
 import contextlib
+import functools
 import imp
 import os
 import os.path
@@ -9,11 +10,24 @@ from test import support
 
 
 def writes_bytecode(fxn):
+    """Decorator to protect sys.dont_write_bytecode from mutation."""
+    @functools.wraps(fxn)
+    def wrapper(*args, **kwargs):
+        original = sys.dont_write_bytecode
+        sys.dont_write_bytecode = False
+        to_return = fxn(*args, **kwargs)
+        sys.dont_write_bytecode = original
+        return to_return
+    return wrapper
+
+
+def writes_bytecode_files(fxn):
     """Decorator that returns the function if writing bytecode is enabled, else
     a stub function that accepts anything and simply returns None."""
     if sys.dont_write_bytecode:
         return lambda *args, **kwargs: None
     else:
+        @functools.wraps(fxn)
         def wrapper(*args, **kwargs):
             to_return = fxn(*args, **kwargs)
             sys.dont_write_bytecode = False