PR Summary

There are a few remaining warnings with secondary_axes, which should be handled by#13593 (which, as a side effect, delays applyings margins to the secondary axes until the axes have "valid" limits).

Edits:

• Looks like test_warn_external_frame_embedded_python had to be slightly adjusted (although it does something nasty by patching sys, so it probably can't really be considered a pytest bug...); done.
• And some warning with old numpys, also fixed.

PR Checklist

• Has Pytest style unit tests
• Code isFlake 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