Add set_cursor method to FigureCanvasTk

[daniilS]

PR Summary

Since 3.5, NavigationToolbar2.set_cursor is deprecated in favour of FigureCanvasBase.set_cursor. This adds the set_cursor method to FigureCanvasTk to make the cursor respond to move/zoom modes again, and removes it from NavigationToolbar2Tk as the parent NavigationToolbar2.set_cursor just calls FigureCanvasBase.set_cursor now.

I'm setting the cursor for the canvas specifically rather than for the canvas's master as before, as the cursor change should only ever affect the canvas (and not the toolbar, or any sibling widgets if dealing with an embedded plot).

PR Checklist

Tests and Styling

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (install flake8-docstrings and run flake8 --docstring-convention=all).

Documentation

• New features are documented, with examples if plot related.
• New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).
• Documentation is sphinx and numpydoc compliant (the docs should build without error).