Add type signature for pyplot.show.

PR summary

This is done to prevent errors when mypy --strict is invoked on client code containing plt.show().
Closes matplotlib#26919.
The signature indicates a return type of None, which matches the matplotlib documentation, even though the implementation could actually return non-None if the back-end function does.

PR checklist

• "closes #0000" is in the body of the PR description to link the related issue
• [] new and changed code is tested
• [N/A] Plotting related features are demonstrated in an example
• [N/A] New Features and API Changes are noted with a directive and release note
• [N/A] Documentation complies with general and docstring guidelines