Log Message:
auctex: update to 13.1

News in 13.1
* In math environments ‘gather’, ‘gather*’, ‘gathered’, \ 
‘multline’ and
  ‘multline*’, fill commands such as M-q and C-c C-q C-e are disabled.
* Now two commands ‘Texindex’ and ‘Texi2dvi’ are available when you type
  C-c C-c in Texinfo mode. The command ‘Texindex’ runs texindex on index
  files and ‘Texi2dvi’ runs pdftexi2dvi or texi2dvi according to the value
  of TeX-PDF-mode.
* AUCTeX’s own help messages for LaTeX errors are now shown only for LaTeX runs.
* Two functions TeX-split-string and TeX-assoc are now obsolete and will be
  removed in future release.
* The function TeX-read-key-val now accepts a function call as second argument.
* AUCTeX now requires GNU Emacs 24.3 or higher.
* Old implementations for viewers were discarded, as announced long before.
* AUCTeX now uses lexical binding which has been introduced in Emacs 24.
* The constant LaTeX-dialect has been renamed to TeX-dialect and moved from
  ‘latex.el’ to ‘tex.el’. LaTeX-dialect now is an obsolete alias.
* The style ‘latexinfo.el’ is removed from AUCTeX.
* The style ‘siunitx.el’ is updated to support package version 3.
* AUCTeX has preliminary support for LaTeX-hooks.
* Many other bugs were fixed.

News in 12.3
* Support for ‘PSTricks’ is now PDF-oriented.
* The function font-latex-update-font-lock has been obsoleted.
* Math expression highlighting was improved.
* AUCTeX tracks changes in LaTeX2e 2020-02-02 release.
* More bugs fixed, other minor features implemented.

News in 12.2
* AUCTeX reflects the changes in LaTeX2e 2019-10-01 release.
* A new method is implemented in preview-latex to adjust the foreground
  colors of generated images to those of Emacs, when the LaTeX command
  produces PDF.
* AUCTeX has support for the Flymake package in Emacs 26 or newer.
* The way the option TeX-record-buffer is used was corrected.
* Several other bugs were fixed, many minor features were added.

News in 12.1
* AUCTeX now requires GNU Emacs 24 or higher. Support for XEmacs has been dropped.