- pyplot.rc_context is a wrapper of matplotlib.rc_context
- pyplot.cm is a re-export of matplotlib.cm
- set the current axes of the Figure in question without affecting global
  current figure

Sniffing for `obj.number` is actually checking for "pyplot managed figures" not
any figure (the attribute gets monkey-patched in as part of creating and
registering the FigureManger via `plt.figure`).  As matplotlib is already
unconditionally imported in this module do the isinstance check directly.

The try...except is to support matplotlib < 3.4 when we added support for
SubFigure.

This enables the user to do:

  sys.modules['matplotlib.pyplot'] = None

to make ensure that they can avoid the global state associated with pyplot if
they wish.