Sasctl

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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.

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**Bugfixes**
- Fixed an issue that caused score code generation by `pzmm` module to fail with Viya 3.5.

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**Bugfixes**
- SSL warnings no longer repeatedly raised when `verify_ssl=False` but `CAS_CLIENT_SSL_CA_LIST` is specified.
- `model_repository.delete_model_contents()` no longer fails when only one file is found.

**Improvements**
- All `delete_*()` service methods return `None` instead of empty string.
- All `get_*()` service methods issue a warning if multiple items are found when retrieving by name.

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**Bugfixes**
- Fixed an import issue that could cause an error while using the `pzmm` submodule.