use conditional requirements for backport libraries Fixes #147

vcr/compat.py

@@ -0,0 +1,18 @@
+try:
+    from unittest import mock
+except ImportError:
+    import mock
+
+try:
+    import contextlib
+except ImportError:
+    import contextlib2 as contextlib
+else:
+    if not hasattr(contextlib, 'ExitStack'):
+        import contextlib2 as contextlib
+
+import collections
+if not hasattr(collections, 'Counter'):
+    import backport_collections as collections
+
+__all__ = ['mock', 'contextlib', 'collections']