Refrain from adding Details to backoff namespace

If needed, I want to make sure we put this in the right place. There
may ultimately be some other types which it would be helpful to expose
as well, and in that case I think it might be better to have a typing
submodule where only these types are exposed.

backoff/__init__.py

@@ -14,7 +14,6 @@
 """
 from backoff._decorator import on_exception, on_predicate
 from backoff._jitter import full_jitter, random_jitter
-from backoff._typing import Details
 from backoff._wait_gen import constant, expo, fibo, runtime
 
 __all__ = [
@@ -26,7 +25,6 @@
     'runtime',
     'full_jitter',
     'random_jitter',
-    'Details',
 ]
 
 __version__ = "2.1.2"