Steampak

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+ Steam API. Added some screenshots related stuff.
* Adopted to a newer ctyped.

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! IMPORTANT: Callback scheduling methods removed as not currently used.
! IMPORTANT: Dropped support for Python < 3.6. Performance improved.
! IMPORTANT: Overlay page constants moved into overlay class.
+ Adapted to 'libsteam_api' version 1.42.

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+ CLI: `get_cards_stats` command now accepts `skip-owned` option.
+ CLI: `get_cards_stats` command now accepts `foil` option.
+ CLI: `get_cards_stats` command now accepts `appid` options.
+ CLI: launcher script is now Windows compatible.
+ Web API: DataFetcher now handles `Too Many Requests` responses.
+ Web API: `Application.get_cards()` now accepts cardboarder filters.
+ `pip install steampak[extra]` now installs extra dependencies (for CLI, Web API)

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* CLI: Fixed get_booster_stats() crash.
* Web API: Fixed encoding problems (e.g. as seen in apps titles).

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* Adds features advertised but missing from 0.4.0

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+ Added mysterious [and sales] cards handling.
+ Some Web API changes adopted.