Could just do `return isinstance(err, ReadTimeoutError) or isinstance(err, HTTPLIB_READ_EXCEPTIONS)`

Or better yet, could rename it to `READ_EXCEPTIONS and add `ReadTimeoutError` to the list, then it's just `return isinstance(err, READ_EXCEPTIONS)`.

At that point, I'd get rid of `_is_read_error(...)` altogether which would make the `increment(..)` code more linear to read. I think this is a good idea in general, to get rid of these one-liner helpers unless they add some other value (ie. reused in other places or useful for overriding). 

Also I'd add `READ_EXCEPTIONS` as a static member of the `Retry` class rather than module-global, so it could be overridden per-instance.