Use python3 as the shebang on Windows

Since commit 013980d8, Python tools like gdbus-codegen, glib-genmarshal,
glib-mkenums, etc. have a shebang with the full path to the python binary.

Shebangs cannot be shell-quoted (most utilities just do not expect that),
but since shebangs can have arguments, the path to the interpreter must not
have spaces. This is a problem on Windows, where Python is commonly installed
under the "Program Files" folder.

Moreover, by using the full path to the interpreter (at build time), the bundle
is not relocatable, which is the norm on Windows. It means that the bundle
cannot be moved between directories or different systems

gio/gdbus-2.0/codegen/meson.build

diff --git a/gio/gdbus-2.0/codegen/meson.build b/gio/gdbus-2.0/codegen/meson.build
index 67ea9f2..a196ea1 100644
--- a/gio/gdbus-2.0/codegen/meson.build
+++ b/gio/gdbus-2.0/codegen/meson.build
@@ -31,7 +31,7 @@
 gdbus_codegen_conf.set('VERSION', glib_version)
 gdbus_codegen_conf.set('MAJOR_VERSION', major_version)
 gdbus_codegen_conf.set('MINOR_VERSION', minor_version)
-gdbus_codegen_conf.set('PYTHON', python.full_path())
+gdbus_codegen_conf.set('PYTHON', host_system != 'windows' ? python.full_path() : 'python3')
 gdbus_codegen_conf.set('DATADIR', glib_datadir)
 
 # Install gdbus-codegen executable

glib/meson.build

diff --git a/glib/meson.build b/glib/meson.build
index 5fd7759..aaa4948 100644
--- a/glib/meson.build
+++ b/glib/meson.build
@@ -501,7 +501,7 @@
 
 report_conf = configuration_data()
 report_conf.set('GLIB_VERSION', glib_version)
-report_conf.set('PYTHON', python.full_path())
+report_conf.set('PYTHON', host_system != 'windows' ? python.full_path() : 'python3')
 configure_file(
   input: 'gtester-report.in',
   output: 'gtester-report',

gobject/meson.build

diff --git a/gobject/meson.build b/gobject/meson.build
index 748f72b..8e9647d 100644
--- a/gobject/meson.build
+++ b/gobject/meson.build
@@ -85,7 +85,7 @@
 
 python_tools_conf = configuration_data()
 python_tools_conf.set('VERSION', glib_version)
-python_tools_conf.set('PYTHON', python.full_path())
+python_tools_conf.set('PYTHON', host_system != 'windows' ? python.full_path() : 'python3')
 
 foreach tool: python_tools
   tool_bin = configure_file(