[gimp-macos-build] jhbuildrc-gtk-osx-gimp: use Python 3.8 instead.

[Jehan <jehanp src gnome org>]

commit 51f116f615543a2b1b22bca1e5bf7ce7cb70e7ad
Author: Jehan <jehan girinstud io>
Date:   Fri Jul 23 21:48:42 2021 +0200

    jhbuildrc-gtk-osx-gimp: use Python 3.8 instead.
    
    gimp-help now sees Python 3 (so the previous commit was right) but it
    fails to see libxml2 from within Python 3.6.
    Since libxml2 seems to be built with Python 3.8.2, let's set this as our
    Python version to use.

 jhbuildrc-gtk-osx-gimp | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)
---
diff --git a/jhbuildrc-gtk-osx-gimp b/jhbuildrc-gtk-osx-gimp
index 8df47f6..7990690 100644
--- a/jhbuildrc-gtk-osx-gimp
+++ b/jhbuildrc-gtk-osx-gimp
@@ -160,8 +160,9 @@ moduleset = 'https://gitlab.gnome.org/samm-git/gtk-osx/raw/gimp/modulesets-stabl
 # force python 3.6 for the libxml2 to avoid build issues
 module_extra_env['libxml2'] = { "ac_cv_path_PYTHON": "python3.6" }
 
-# force python 3.6 for gimp-help.
-module_extra_env['gimp-help-git'] = { "PYTHON": "python3.6" }
+# force python 3.8 for gimp-help because libxml2 is built with Python
+# 3.8 support.
+module_extra_env['gimp-help-git'] = { "PYTHON": "python3.8" }
 
 # meson needs UTF-8 charset for build python scripts to work correctly
 module_extra_env['meson'] = { "LC_ALL": "en_US.UTF-8" }