PR Summary

Something trivial I found while scrolling around making sure I covered every join/capstyle we allow. Currently, collections.py, rcparams.py, and lines.py all have their own way of checking whether a cap/join-style is valid (patches.py uses the code from lines.py). These all boil down to a cbook._check_in_list, but some forget to warn that case-insensitivity is deprecated.

Since rcparams.py seems like the centralized place for checking that parameters are valid, I had them all call rcparams.validate_[cap|join]style() instead. This correctly warns for deprecated case-insensitivity.

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