[gimp] meson: use explicitly python3 when calling it.

[Jehan <jehanp src gnome org>]

commit 44df9f219318e7fcb7b0d9b82f47ec91bb710f68
Author: Jehan <jehan girinstud io>
Date:   Sun Nov 24 16:47:55 2019 +0100

    meson: use explicitly python3 when calling it.
    
    Anyway meson is based itself on python3 so it has to be present. Just
    using `python` may be any python (2 included).
    
    Of course, it would be ok most of the time, but with the Fedora 31 CI,
    apparently just `python` is not found.

 meson.build | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/meson.build b/meson.build
index c28bf08eab..4779dcfb11 100644
--- a/meson.build
+++ b/meson.build
@@ -16,7 +16,7 @@ conf = configuration_data()
 warnings = []
 
 # git-version.h is already present and not generated if dist tarball
-is_git_repository = run_command('python', '-c',
+is_git_repository = run_command('python3', '-c',
   'import sys,os; sys.exit(0 if os.path.exists(".git") else 1)'
 ).returncode() == 0
 
@@ -1431,7 +1431,7 @@ endif
   #####   ####  #    # #      #  ####     #       # ###### ######  ####
 
 
-has_version_h = run_command('python', '-c',
+has_version_h = run_command('python3', '-c',
   'import sys,os; sys.exit(0 if os.path.exists("git-version.h") else 1)'
 ).returncode() == 0
 if is_git_repository or not has_version_h