Trading-ig

* simplified request headers: `requests.Session` now handles persistence
* adds support for v3 session creation (157)
* implementation and test for fetch_working_orders() v2 (like PR187)
* adds support for fetch_historical_prices_by_epic_and_date_range() v2
* adds support for fetch_open_positions() v2
* dependency management with [Python Poetry](https://python-poetry.org/) (#149)
* more robust handling of IG API rate limits
* support for all the `/history/activity` endpoints
* better test coverage. now 80% for `rest.py` (38)
* documentation improvements

* integration and unit tests improved
* remove reference to adapter (190)
* pandas 1.2.0 (184)
* paged historical data can be captured with one request (183)
* paging fixed for historical data (175)
* error fixed when closing stream (174)

* docs restructured and reintegrated with readthedocs.io
* Fix for a bug in 'expand-columns()' that would cause an error if values were missing/null
* unit tests for v3 historic prices method
* Tidy up usage of "version" across the REST API to make it consistent
* Marked methods that use an outdated version with TODOs
* Fixed issues with switch_account(), delete_working_order() etc. which were not specifying a version correctly
* unit tests for v1, v2 historic prices methods
* allowing historic prices format to be defined at runtime
* new optional flatter format for historic prices
* release notes

* CI build fixed
* scheduled integration test added
* initial unit tests created for login, utils
* historical prices methods fixed to use NaN instead of None
* requirements defined explicitly
* fixing flake8 errors
* fixed stream_connection bug
* fixed encrypted password bug
* fixed streaming LOOP problem
* fixed various API version issues
* fixed missing Crypto dependency issue
* fixed Pandas version dependency issue
* support removed for older python versions
* formatting with black
* fixing date format issues
* Fetch multiple client sentiment markets
* PEP8 improvements
* Improve not-found handling on working order creation
* Break out of accounts loop when a user is found
* rest: create_working_order upgrade to v2 api
* Add fetch transaction history v2 api
* Add systemd watchdog support to the lightstreamer library
* Add method to GET current session details
* historical prices v3
* better exception handling
* Update lightstreamer library

* initial / undocumented