- 3D Axes are created via add_subplot(projection='3d')
- There is now a `stairs()` function that's specifically designed for
  showing histogram curves
- Labels should be passed as keyword arguments to the plot functions
  instead of to `legend()`, which reduces the risk of mixing them up.
- ensure equal axis scaling in the meshgrid example

* Cleanup unused imports (F401) of mostly standard Python modules,
  or some internal but unlikely referenced modules
* Where internal imports are potentially used, mark with noqa
* Avoid redefinition of imports (F811)

This absolute beginners tutorial is the output of Anne's Google
Season of Docs project. An intermediate version was also published at
https://towardsdatascience.com/the-ultimate-beginners-guide-to-numpy-f5a2f99aef54