wrapper: replace usage of deprecated imp module for Python 3

A warning is emitted

  DeprecationWarning: the imp module is deprecated in favour of
  importlib; see the module's documentation for alternative uses

Change-Id: I6c5a9e024a9a904e02a24331f615548be3fe5f8e
Signed-off-by: David Pursehouse <dpursehouse@collab.net>
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/230984
Reviewed-by: Jonathan Nieder <jrn@google.com>

wrapper.py

@@ -15,7 +15,12 @@
 # limitations under the License.
 
 from __future__ import print_function
-import imp
+try:
+   from importlib.machinery import SourceFileLoader
+   _loader = lambda *args: SourceFileLoader(*args).load_module()
+except ImportError:
+   import imp
+   _loader = lambda *args: imp.load_source(*args)
 import os
 
 
@@ -26,5 +31,5 @@ _wrapper_module = None
 def Wrapper():
   global _wrapper_module
   if not _wrapper_module:
-    _wrapper_module = imp.load_source('wrapper', WrapperPath())
+    _wrapper_module = _loader('wrapper', WrapperPath())
   return _wrapper_module