LaTeX: use current parskip package for better spacing around titles (#12833)

Co-authored-by: Adam Turner <9087854+aa-turner@users.noreply.github.com>

CHANGES.rst

@@ -6,6 +6,8 @@ Dependencies
 
 * #12756: Add lower-bounds to the ``sphinxcontrib-*`` dependencies.
   Patch by Adam Turner.
+* #12833: Update the LaTeX ``parskip`` package from 2001 to 2018.
+  Patch by Jean-François B.
 
 Incompatible changes
 --------------------

sphinx/texinputs/sphinxlatexstylepage.sty

@@ -4,13 +4,7 @@
 \ProvidesPackage{sphinxlatexstylepage}[2021/01/27 page styling]
 
 % Separate paragraphs by space by default.
-\IfFileExists{parskip-2001-04-09.sty}% since September 2018 TeXLive update
-% new parskip.sty, but let it rollback to old one.
-% hopefully TeX installation not broken and LaTeX kernel not too old
-   {\RequirePackage{parskip}[=v1]}
-% standard one from 1989. Admittedly \section of article/book gives possibly
-% anomalous spacing, but we can't require September 2018 release for some time.
-   {\RequirePackage{parskip}}
+\RequirePackage{parskip}
 
 % Style parameters and macros used by most documents here
 \raggedbottom