Refactor sender module

[felix-hilden]

From #131. We can't refactor the sender module yet, but I have an idea for an improvement. It would break setting defaults though. We could divide the module into submodules like this:

• sender.base: Sender, Async/Sync Sender
• sender.concrete: Transient, Persistent and Singleton sync/async senders
• sender.extend: Retrying / other extensions that we come up with
• sender.default: kwargs / sender instance / sender type

This would actually improve setting defaults. Instead of two imports, we could do one:

# Old
from tekore import sender
from tekore.sender import PersistentSender

sender.default_sender_type = PersistentSender

# New
from tekore.sender import default, PersistentSender

default.sender_type = PersistentSender
default.requests_kwargs = {}

I realise the old example could be done with one import, but it would lead to longer names. And now there is no problem importing defaults in other sender submodules. The client base class could also be in e.g. sender.client or base.

[felix-hilden]

With the changes in 2.0 this change won't matter to the users, but let's still do it for the refactoring's sake.