Plasticscm

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- Add support for Python 3.9.
- General update, improvements and cleanup.
- Setup update and cleanup.
- Fixed docs setup.

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- Drop support for Python 3.5
- Added get_cm_location().
- | Forced 'query' to be a keyword-only argument for get_branches(),
| get_labels(), get_changesets().
- Forced 'top_level' to be a keyword-only argument for create_branch().
- Fixes for get_pending_changes() and undo_pending_changes().
- Added ReadTheDocs config file.
- Setup update.

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- Added get_item_revision().
- Docstings have been completed.
- Changes for preliminary tests.
- Added a preliminary documentation.

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- Fixes for rename_repository() and rename_workspace().
- Changes of OperationStatus and CheckinStatus models.
- Added optional parameter 'timeout'.
- Fixes for undo_pending_changes().
- Fix for typos: delete_worskpace -> delete_workspace.
- Fixes for get_item_revision_history_*().
- Added a preliminary tests.

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- First alpha release.

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- Initial commit.