Sasctl

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**Improvements**
- Added Git integration for better tracking of model history and versioning.
- Added MLFlow integration for simple models, allowing users to import simple MLFlow models, such as sci-kit
learn, to SAS Model Manager

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**Bugfixes**
- Fixed an issue where `folders.create_folder()` would attempt to use root folder as parent if desired parent
folder wasn't found. Now correctly handles parent folders and raises an error if folder not found.

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**Bugfixes**
- Fix an issue where `pzmm.ZipModel.zipFiles()` threw an error on Python 3.6.1 and earlier.

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**Bugfixes**
- Fixed an issue with `register_model()` where random forest, gradient boosting, and SVM regression models with
nominal inputs where incorrectly treated as classification models.

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**Improvements**
- `model_repository.add_model_content()` will now overwrite existing files instead of failing.

**Bugfixes**
- `PagedList.__repr__()` no longer appears to be an empty list.

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**Improvements**
- `Session` now supports authorization using OAuth2 tokens. Use the `token=` parameter in the constructor when
an existing access token token is known. Alternatively, omitting the `username=` and `password=` parameters
will now prompt the user for an auth code.

**Changes**
- `current_session` now stores & returns the *most recently created* session, not the first created session. This
was done to alleviate quirks where an old, expired session is implicitly used instead of a newly-created session.
- Removed deprecated `raw=` parameter from `sasctl.core.request()`.
- Dropped support for Python 2.