Let kraken.spot.OrderbookClient use Spot Websocket API v2

[btschwertfeger]

Summary

The Spot orderbook client was upgraded to use the Kraken Spot websocket API v2.

This is a breaking change, since the raw messages sent by Kraken will be forwarded (as before) to the on_book_update function- BUT they are now in a different format.

https://docs.kraken.com/websockets-v2/#book

[codecov]

Codecov Report

Merging #139 (09ddfc1) into master (16627d8) will decrease coverage by 0.06%.
The diff coverage is 94.59%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #139      +/-   ##
==========================================
- Coverage   90.05%   90.00%   -0.06%     
==========================================
  Files          20       20              
  Lines        1720     1711       -9     
==========================================
- Hits         1549     1540       -9     
  Misses        171      171              

Flag	Coverage Δ
unittests	90.00% <94.59%> (-0.06%)	⬇️

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

Files Changed	Coverage Δ
kraken/spot/websocket_v2.py	96.77% <50.00%> (ø)
kraken/spot/orderbook.py	89.02% <96.42%> (-1.09%)	⬇️
kraken/spot/funding.py	72.97% <100.00%> (ø)
kraken/spot/market.py	100.00% <100.00%> (ø)
kraken/spot/staking.py	75.00% <100.00%> (ø)
kraken/spot/trade.py	94.28% <100.00%> (ø)
kraken/spot/user.py	99.12% <100.00%> (ø)
kraken/spot/websocket/connectors.py	79.34% <100.00%> (ø)