plot_directive: Avoid warning if plot_formats doesn't contain 'png'

[mgeier]

PR Summary

When using matplotlib.sphinxext.plot_directive in Sphinx and for example using this setting:

plot_formats = ['svg', 'pdf']

... or anything without 'png' for that matter, I get a warning for each plot:

WARNING: image file not readable: _build/plot_directive/index-1.png

This is caused by the texinfo section in the template, which seems to expect a .png file no matter what.

This PR includes the PNG image only if PNG is among the chosen formats.

This is the same fix as #3530 did to fix issue #2800 for the PDF case.

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way