configure_subplots now needs to importpyplot (because that's
how we can spin up a new figure as needed), but when relying on
embedding we could well reach that point with an already running
interactive framework, pyplot never imported, and an incompatible
rcParams["backend"] (which had not been read yet). For example, run
examples/user_interfaces/embedding_in_tk_sgskip.py with a non-tk
backend set in your matplotlibrc: clicking on the the configure_subplots
button triggers a "cannot load backend 'foo' which requires the 'foo'
interactive framework, as 'bar' is currently running".

Instead, use a helper to safely import pyplot when another interactive
framework may already be running. The list of frameworks that
_get_running_interactive_framework returns is fixed, so we can just
hardcode the mapping. (We are going to ignore the user-set
rcParams["backend"] anyways.)

followup to#16818, especially#16818 (review) (which this does not fix, it's just the easy first step).

PR Summary

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/next_api_changes/* if API changed in a backward-incompatible way