Adjusted FileFinder import to work with Python 3.6 (#318)

In CPython 3.6, FileFinder has been moved from importlib._bootstrap to
a new importlib._bootstrap_external module.

To account for this, this commit switches to importing the more general
importlib.machinery module instead of importlib._bootstrap, which also
works with older CPython version (3.3+).

pex/finders.py

@@ -22,9 +22,9 @@
 import pkg_resources
 
 if sys.version_info >= (3, 3) and sys.implementation.name == "cpython":
-  import importlib._bootstrap as importlib_bootstrap
+  import importlib.machinery as importlib_machinery
 else:
-  importlib_bootstrap = None
+  importlib_machinery = None
 
 
 class ChainedFinder(object):
@@ -214,8 +214,8 @@ def register_finders():
   # append the wheel finder
   _add_finder(pkgutil.ImpImporter, find_wheels_on_path)
 
-  if importlib_bootstrap is not None:
-    _add_finder(importlib_bootstrap.FileFinder, find_wheels_on_path)
+  if importlib_machinery is not None:
+    _add_finder(importlib_machinery.FileFinder, find_wheels_on_path)
 
   __PREVIOUS_FINDER = previous_finder
 
@@ -230,8 +230,8 @@ def unregister_finders():
   pkg_resources.register_finder(zipimport.zipimporter, __PREVIOUS_FINDER)
   _remove_finder(pkgutil.ImpImporter, find_wheels_on_path)
 
-  if importlib_bootstrap is not None:
-    _remove_finder(importlib_bootstrap.FileFinder, find_wheels_on_path)
+  if importlib_machinery is not None:
+    _remove_finder(importlib_machinery.FileFinder, find_wheels_on_path)
 
   __PREVIOUS_FINDER = None