Landscape-api-py3

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- fixed documentation

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- fix 245: CreateScript endpoint raises exception (solution suggested and tested)
- fix 246: CreateScriptAttachment endpoint raises exception (solution suggested and tested)
- minor format change
- add pyproject.toml

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- fix 19
- minor updates

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* added support for Python 3.5, 3.6, 3.7, 3.8 and above
* fixed minor bugs
* updated documentation (Installation, Usage, Quick start)

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* fixed documentation import bug
* fixed default CA cert bug

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* fixed bug with imports - now it's compatible with Canonical **landscape-api**