Use partialmethod for better signatures in backend_ps.

[anntzer]

I missed this possibility in 26253f7, but the idea is the same (see motivation there).

Move format to be the first parameter of _print_ps and friends, to
make the partialmethods easier to define. Also rename format to fmt
while we're at it, to avoid shadowing the builtin format function.

PR Summary

PR Checklist

Tests and Styling

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (install flake8-docstrings and run flake8 --docstring-convention=all).

Documentation

• New features are documented, with examples if plot related.
• New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).
• Documentation is sphinx and numpydoc compliant (the docs should build without error).