Depending on the version of luatex, the pgf backend would either emit
\pdfpagewidth or \pagewidth. Instead of explicitly checking the version
of luatex (which requires additional code to parse the output of
--version, and is brittle to (admittedly unlikely) possible future other
changes in luatex or xetex), just emit some TeX code that emits
whichever primitive is defined.

(https://tex.stackexchange.com/a/31658/4101 for the terrifying TeXnical details)

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is PEP 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