Apply hints suggested by the ruff linter

[btschwertfeger]

Summary

Some more ruff rules were enabled which only affects/improves the unit tests.

Also the documentation was somewhat extended for the example scripts of this repositroy.

[codecov]

Codecov Report

Merging #165 (f269b1b) into v2 (8072cb9) will not change coverage.
Report is 1 commits behind head on v2.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##               v2     #165   +/-   ##
=======================================
  Coverage   90.52%   90.52%           
=======================================
  Files          21       21           
  Lines        1836     1836           
=======================================
  Hits         1662     1662           
  Misses        174      174           

Flag	Coverage Δ
unittests	90.52% <100.00%> (ø)

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

Files	Coverage Δ
kraken/base_api/__init__.py	89.94% <100.00%> (ø)
kraken/exceptions/__init__.py	99.18% <ø> (ø)
kraken/futures/market.py	91.66% <ø> (ø)
kraken/futures/trade.py	96.20% <100.00%> (ø)
kraken/futures/websocket/__init__.py	78.61% <ø> (ø)
kraken/futures/ws_client.py	87.50% <100.00%> (ø)
kraken/spot/orderbook_v1.py	94.44% <ø> (ø)
kraken/spot/orderbook_v2.py	94.04% <100.00%> (ø)
kraken/spot/trade.py	94.33% <100.00%> (ø)
kraken/spot/user.py	99.12% <100.00%> (ø)
... and 2 more