Integrate Kraken Websockets API v2; add kraken.spot.KrakenSpotWebsocketClientV2; internals

[btschwertfeger]

Summary

This PR provides a new client that can be used to connect to the Kraken Websocket API V2 (kraken.spot.KrakenSpotWSClientV2).

To do so, new base classes were used to support both API v1 and v2.

Breaking

• Some internals have changed. This only affects users that used non-public functions or classes that were not documented so far.
• The parameter msg of kraken.spot.KrakenSpotWSClient.on_message was renamed to message.
• The parameter msg of kraken.spot.KrakenSpotWSClient.send_message was renamed to message.

Todo's / Changes

• add kraken.spot.KrakenSpotWSClientV2 and create a base class to support API v1 and v2
• adjust the examples (/examples/*.py and README.md)
• adjust the documentation and add new / remove old stuff
• write new tests for Websocket API V2
• use __all__ in python modules
• do we still need to ping? - Yes - the Kraken Docs are recommending it.

[codecov]

Codecov Report

Merging #131 (17a6e2a) into master (9f30c1b) will increase coverage by 2.42%.
The diff coverage is 91.37%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #131      +/-   ##
==========================================
+ Coverage   87.57%   90.00%   +2.42%     
==========================================
  Files          18       20       +2     
  Lines        1569     1720     +151     
==========================================
+ Hits         1374     1548     +174     
+ Misses        195      172      -23     

Flag	Coverage Δ
unittests	90.00% <91.37%> (+2.42%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
kraken/spot/websocket/connectors.py	79.34% <79.34%> (ø)
kraken/spot/orderbook.py	90.10% <95.45%> (+0.22%)	⬆️
kraken/spot/websocket_v2.py	96.77% <96.77%> (ø)
kraken/spot/websocket/__init__.py	96.96% <96.82%> (+21.81%)	⬆️
kraken/spot/websocket_v1.py	97.16% <98.38%> (ø)
kraken/base_api/__init__.py	90.05% <100.00%> (-0.43%)	⬇️
kraken/exceptions/__init__.py	99.20% <100.00%> (+0.01%)	⬆️
kraken/futures/__init__.py	100.00% <100.00%> (ø)
kraken/futures/funding.py	64.00% <100.00%> (+3.13%)	⬆️
kraken/futures/market.py	91.39% <100.00%> (+0.18%)	⬆️
... and 10 more