Trading-ig

* fix for http error 405 response from IG when confirming deal (PR237)
* updating Poetry for automated testing
* removing some unused legacy project config
* removing support for unsupported Python 3.6, adding 3.10
* better handling and docs for optional dependencies (216, 240)
* fixing occasional KeyError when deleting session headers (238)

* optional dependency `tenacity` updated to latest
* documentation improvements
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* fixes the bug where guaranteed stop loss was replaced with a normal stop loss on working order update (224)
* legacy 'setup.py' style project config files removed

* switch accounts bug fixed (220)
* coverage badge and report added
* Sphinx is now a development dependency (228)
* test improvements

* uses Poetry to publish to [Python Package Index](https://pypi.org/)

* code samples improved, new FAQ
* Adds support for update open position v2
* new function to calculate mid prices for historic data
* Adds support for getting and updating account prefs
* Adds support for `/markets` endpoint for getting details of multiple markets
* new sample script to traverse navigation tree and get epics, new FAQ