Compare e.g. an excerpt of the docs of matplotlib._tri.Triangulation
before

 |  calculate_plane_coefficients(...)
 |      calculate_plane_coefficients(z, plane_coefficients)
 |
 |      Calculate plane equation coefficients for all unmasked triangles
 |
 |  get_edges(...)
 |      get_edges()
 |
 |      Return edges array

and after

 |  calculate_plane_coefficients(z, plane_coefficients)
 |      Calculate plane equation coefficients for all unmasked triangles
 |
 |  get_edges()
 |      Return edges array

This means that inspect.signature() now knows about the correct
signature, and signature tests can use inspect.signature().bind rather
than matching on the error string.

The convention of using "func_name(arg, ...)--\n\n" for
C-level signatures is admittedly undocumented, but comes from
https://bugs.python.org/issue20530.

Inspired by the need to update error message strings in #14964 (although this would not have helped because Py2 doesn't support inspect.signature, but heh).

PR Summary

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